Study Guide Zoology Exam 3

Acoelomate definition

Animals that lack a body cavity (coelom) between the digestive tract (gut) and outer body wall

What is triploblastic body plan?

Organisms that have 3 embryonic layers consisting of the ectoderm, the outermost layer, the mesoderm, the middle layer, and the endoderm which is the innermost layer.

Which phylum possesses a more complex level or organization? Cnidarians or Platyhelminthes

Platyhelminthes

What clade does the Platyhelminthes belong to?

Lophotrochozoa, a major group within the protostomes

How many species are there within the Platyhelminthes?

Approximately 20,000 species

Basic Characteristics for the Platyhelminthes

• Dorsally flattened

• Has bilateral symmetry

• Unsegmented

• Acoelomate

• Triploblastic

What type of digestive system do the Platyhelminthes have?

They have an incomplete or absent gut

What type of nervous system do the Platyhelminthes have?

They have an anterior ganglia (primitive brain) and longitudinal nerve cords with transverse connections that appear ladder-like.

What type of reproduction do the Platyhelminthes have?

They are monoecious (hermaphroditic

Classes of Platyhelminthes

Turbellaria: most are free living flatworms that are aquatic (ex. Planaria)

Monogenea: they are external parasites (mainly on fish) that have a simple life cycle

Trematoda: They are internal parasites (flukes) that have a complex life cycle involving intermediate hosts.

Cestoida: Tapeworms essentially, which are intestinal parasites of vertebrates that have no digestive system and absorb nutrients directly from the host.

General Characteristics for Turbellaria

• There are 3,000 species

• Mostly free living and are found in freshwater, saltwater, and a few terrestrial environments

• Carnivorous

• Size can range from 1 cm to 60 cm

Platyhelminthes: Turbellaria Body Wall Composition

Ectodermal: Originates at the epidermis which has microvilli and is ciliated

Mesodermal: Originates at the muscle layers which are circular, longitudinal, and dorsoventral. It can also originate at the parenchymal cells which fills up space and support organs.

Endodermal: Originates at the gastrodermis which lines the gut/digestive cavity (digestive cavity secretes enzymes and absorbs nutrients)

Platyhelminthes: Turbellaria Body Structure Ventral Surface Features

• Most organs are close to the ventral body wall

• Ventral Surface has ventral nerve cords

• The Epidermal structures include Rhabdites, adhesive and releaser glands, and cilia

• Rhabdites: Mucus secreting rod-shaped cells

• Adhesive and releaser glands: Aid in locomotion

• Cillia: For gliding movement

Platyhelminthes: Tubellaria Locomotion

They can move via creeping and swimming.

Creeping: cilia covered epidermis and mucus sheet (head first) causes turbulence in the water. Beatings of the cilia propels the worm forward and the mucus sheet helps reduce friction on substrates. There is also muscle contraction involved in providing control and direction.

Swimming: less common, involves the undulation and contraction of the muscle to thrust against the water allowing for smooth wavelike motions through the water.

Platyhelminthes: Turbellaria Feeding Diet

They mostly feed on small invertebrates, dead larger organisms, and algae

Platyhelminthes: Turbellaria Digestive Systems

They have a blind gut meaning they only have one opening that serves as a mouth and they do not have an anus. Digestion begins extracellularly in the gut cavity and complete intracellularly via phagocytosis. They have a pharynx that is used to ingest food and excrete waste. Once food enters the pharynx it passes to the gastrovascular cavity where digestion and nutrient absorption occur. It also takes place of a circulatory system.

Platyhelminthes: Turbellaria Groups based on gut complexity

Acoela: no gut or very simple

Tricladia: three branched gut

Polycladia: multi branched gut

Intestinal branches spread throughout the body, allowing for efficient distribution of nutrients, if there is increased complexity in the branches, there is an increase in area of digestion and absorption meaning they can digest larger amounts of food.

Platyhelminthes: Turbellaria Osmoregulation

This is essential for maintaining water and ion balance by removing excess water and nitrogen waste (ammonia) especially because many species live in freshwater or marine environments. Freshwater species take in water by osmosis and must expel it in order to not burst, while marine species are isotonic. Isotonic refers to their ability to maintain an equal concentration of solutes inside and outside their cells.

Platyhelminthes: Turbellaria Exchanges with the environment

Gas exchange: via simple diffusion

Metabolic Waste Removal: via simple diffusion

What is the protonephridia in the phylum Platyhelminthes class Turbellaria?

Part of the osmoregulation process. The primary osmoregulatory organs in turbellarians. They consist of networks of tubules ending in specialized flame cells with cilia that create flame-like movements to draw in fluid into the system. The network of tubules transports the filtered fluid away from the flame cells. The nephridiopores (fenestrations) are openings through which fluid is expelled from the body.

Platyhelminthes: Turbellaria Nervous System

Acoela: Subepidermal nerve plexus (nerve net) and has statocysts for balance

Tricladida: They have a centralized cerebral ganglia and a distinct head region with paired longitudinal nerve cords that are connected by commissures, creating a ladder like nerve structure and lateral nerve branches.

Platyhelminthes: Turbellaria Sensory System

• Touch Sensors (tactile receptors): distributed throughout the body, composed of sensory cells, help the organism navigate surfaces, avoid predators, and detect prey.

• Chemical (Olfactory) Receptors: function is to detect dissolved chemicals, food sources, toxins. Primarily found on the head (usually around auricles if present)

• Pressure Sensors: Function to detect water currents, pressure changes, or fluid movement around the body.

• Statocysts: Detect gravity and help with body orientation. A fluid filled sac containing granules that is surrounded by cilia.

• Ocelli (simple eyespots): Function to detect light intensity and direction.

• Negative Phototaxis: Turbellarians are usually negatively phototactic, meaning they move away from light. This behavior protects them from UV damage and desiccation.

Platyhelminthes: Turbellaria Reproduction

Exhibit both asexual and sexual reproduction.

• Asexual Reproduction includes transverse fission where the body divides into 2 parts along a transverse plane (across the body). The result is 2 genetically identical individuals which is a form of clonal reproduction.

  • Asexual reproduction produces zooids, which are genetically identical individuals that may remain attach to the parent organism or live independently. Zooids can form colonies that live in a coordinated manner.

• Sexual reproduction includes knowing that these species are monoecious (have separate sexes and reproductive organs). It includes the cross fertilization of the male and female gonads (although some species can self fertilize). Eggs are encased into a cocoon where development can be direct growth or involve larval metamorphosis.

  • Male has testis, sperm duct, seminal vesicles, penis, genital chamber

  • Female has ovary, oviduct, vitellaria, copulatory sac, and genital pore.

Platyhelminthes Monogenea Overview

Type of Parasite: Ectoparasite

Primary Host: Fishes

• They attach onto the gills of fishes to feed the organism’s epithelial tissue, mucus, and blood.

• They attach to the host using their opisthaptor which is a posterior attachment organ.

Platyhelminthes Monogenea: Life cycle

They have a single generation where the life cycle involves only one generation of individuals per host.

Life Cycle Process:

1. Ciliated Larvae: The larvae swim freely in the water

2. Attachment to New Hosts: The larvae finds a new host (typically a fish) where they attach using the opisthaptor.

3. Mature into Adults: Once attached to the host, the larvae develop into adults and begin to feed on the host’s tissues

4. Reproduction: reproduce directly on the host with no intermediate hosts and the life cycle repeats.

Platyhelminthes Trematoda (flukes) Overview

• Has 8,000 species

• As adults they are all endoparasites

• Usually range from 1mm to 6cm in size

• They are flukes

• Their host in their adult form are usually vertebrates

Platyhelminthes: Trematoda Tegument (Body Wall)

The tegument is the specialized body wall that helps trematodes survive their parasitic lifecycle. It serves as a protective covering for feeding on host cells, protection from the host immune system, and absorption of nutrients.

Structure:

• Outer Zone

  • Glycocalyx: A sugar rich coating

  • Microvilli: Finger like projections on the outer surface for absorption and attachment

  • Base membrane: Supports the outer layer

  • Cytoplasmic Bridge: Connects outer zone to inner zone

• Inner Zone

  • Contains the nucleus and most organelles required for cellular functions.

Platyhelminthes: Trematoda Body Plan

External:

• Oral Suckers: Used for attachment to the host and feeding

• Mouth: Opening for ingestion of food.

• Pharynx: Muscular tube that moves food from mouth to the digestive system.

• Esophagus: Connects mouth to digestive cavity

• Acetabulum: A second sucker used for attachment

Internal:

• Excretory Vesicle: Stores excretory waste

• Nephridiopore: Opening for waste elimination

• Reproductive Organs

  • Ovary, Oviduct, Uterus

  • Vitelline Gland: produces eggs

  • Testes, Seminal Vesicle, Vas Deferens

Platyhelminthes: Trematoda What is Complex Life Cycles (general)

At least 2 hosts are required for the parasite to complete its life cycle. Usually one intermediate host (often an invertebrate) where the larval stage develops and a definitive host (a vertebrate) where the parasite reaches sexual maturity.

Platyhelminthes: Trematoda Life Cycle of Schistosome Fluke

1. Egg is released by adult flukes (can be released through feces and urine)

2. The egg hatches into a miracidium that infects a snail by penetrating snail tissue

3. Sporocyst develop within the snail

4. The cercariae is released by the snail into the water and enters the human host directly through penetrating the skin, matures into adults in the human bloodstream, and reproduce in the liver.

Platyhelminthes: Trematoda Life Cycle of Clonorchis Liver Fluke

1. Egg is released into the environment by the adult fluke and is passed through the feces of an infected

2. The free swimming larva stage hatches from the eggs known as miracidium

3. The miracidium infects a snail host when it becomes ingested, where it develops into a sporocyst

4. The sporocyst produces redia, which in turn produce cercaricae

5. The ceracia exits the snail

6. The cericaria infects a fish, encysting in its tissues as a metacericaria

7. The metacericaria in the fish skin is eaten by a human, where it matures into an adult fluke in the liver.

Platyhelminthes: Trematoda General Life Cycle

There are 2 main forms of this Life cycle, the adult form, where they reproduce, and the larval stages.

They have 2 hosts which are the definitive host in which the parasite reaches adulthood and reproduces, and the intermediate host in which the larval form develops.

Platyhelminthes: Cestoidea Overview

Are essentially tapeworms.

• 3,500 species

• All of them are endoparasites

• They live in the digestive tracts of vertebrates

• They can range from 1mm to 25 meters

• They do not have a mouth or digestive tract and the absorb nutrients directly through their tegument

• They have a complete set of reproductive organs

Platyhelminthes: Cestoidea Body Structure

Scolex: Head region, used for attachment (has suckers or hooks)

Neck: Region where new body segments are generated

Proglottids (strobila): Repeating segments that make up most of the body and each proglottid contains a complete set of reproductive organs.

Tegument: This is their body wall that is specialized for nutrient absorption and protects the parasite against the digestive enzymes of the host.

Platyhelminthes: Cestoidea Organ Systems (simplified)

Digestive System: Absent, they rely entirely on absorption through the tegument

Nervous System: Consists of ganglia (cluster of nerve cells) and 2 lateral nerve cords

Excretory System: protonephridial system, removes waste using flame cells.

Platyhelminthes: Cestoidea Reproduction

They are monoecious where each proglottid contains both male and female reproductive organs.

Types of Proglottids:

• Immature Proglottids: near the neck, still developing

• Mature Proglottids: contain function reproductive organs

• Gravid Proglottids: filled with fertilized eggs that break off and exit the host in feces.

Fertilization can occur between proglottids of the same tapeworm or between different tapeworms

(meaning if you have 1 tapeworm you likely have multiple more)

Platyhelminthes: Cestoidea Life Cycle (Beef Tapeworm)

1. Gravid proglottids or eggs exit the human host via feces

2. Cattle ingest eggs from contaminated grass or water

3. Eggs hatch in the cow’s intestine into larvae that are called oncospheres

4. Oncospheres penetrate the intestinal wall and migrate to muscles, where they form cysticerci (encysted larvae)

5. Humans become infected by eating undercooked beef containing the cysticeri

6. Inside the human intestine, cysticeri develop into tapeworms, completing its life cycle.

Key characteristics of the tapeworm

• Their entire body is built for reproduction and nutrient absorption

• They do not have a digestive system and rely on the host

• Infection usually involves multiple worms

• Highly specialized attachment structures (hooks, suckers, on scolex)

Platyhelminthes: Nemertea Overview

• Common Name: Ribbon worms

• 900 species

• Mostly marine and could be found in sand and mud

• Body form:

  • Elongated and flattened

  • Millimeter to several centimeters in size (some can be larger)

  • Unsegmented

  • Bilaterally symmetrical

  • Triploblastic (3 germ layers)

  • Acoelomate

Platyhelminthes: Nemertea Body Systems

• Digestive System: complete digestive tract with a separate mouth and anus

• Nervous System: Cephalization is the concentration of sense organs at the head. They have a cerebral ganglion which is a brain-like structure, longitudinal nerve cords, and transverse commissures that connect the nerve cords.

• Excretory System: Uses protonephridia (flame cell based filtration)

• Circulatory System: Closed circulatory system (unlike flatworms, which lack one)

Platyhelminthes: Nemertea Proboscis

Their very own unique feeding organ that can be turned inside out, is often compared to a spear gun, contains a stylet (sharp tip) to stab and capture prey, housed in a fluid felled cavity called the rhynchocoel, can be rapidly extended to stab prey, and they are also carnivorous.

Platyhelminthes: Nemertea Reproduction and Development

• They are dioecious meaning they have separate male and female sexes

• They reproduce externally

• They have a larval stage known as the pilidium larva, they have a short larval period before settling and developing into adults.

Platyhelminthes: Nemertea Locomotion

They move using the cilia to glide along a mucus trail, their muscles contract for crawling or subtle body movements.

Body Plan for members of the Phyla Lophotrochozoa (Rotifera) and Phyla Ecdysozoa (Nematoda)

Supraphyla:

• Lophotrochozoa: Do not molt, may have lophophores or trochophore larvae.

• Ecdysozoa: Animals that molt (shed their cuticle)

Body Plan:

• Coelomate (pseudocoelomate): Not fully lined with mesoderm

• Triploblastic, and has bilateral symmetry

• Unsegmented

• Constant Cell number

• Muscular Pharynx

• Has a complete digestive tract

• Protonephridia or Renette System

• Cuticle present

• Has adhesive glands

• No circulatory or gas exchange system

Coelomate definition

A coelomate is an animal that has a true coelom, which is a fluid filled body cavity completely lined by mesodermal tissue (middle germ layer). This develops from the blastocoel and the muscles and organs are in direct contact with coelomic fluid. This allows for complexity in organ development and efficient circulatory and excretory systems.

Advantages for the Nematoda and Rotifera of having a complete gut

• They are the first group with a complete gut

• Food is able to enter one end and exit through the other

• A complete gut allows for specialization of regions and conditions in the gut

• It also allows for more efficient digestion

What is a cuticle? (In Nematodes and Rotifera)

Important external structure that serves as a protective and supportive outer covering. It is secreted by the syncytial epidermis, it may have spines, scales, or projections, and some species molt the cuticle to grow.

Nematoda Cuticle

The nematode cuticle is a thick, multilayered, non-cellular covering. It is made of collagen that protects the organism and acts as an external skeleton to maintain the worm’s shape and internal pressure and aids in locomotion. They will also molt their cuticle several times during development.

• Contains 3 layers: cortex, matrix, basal layer

• Continues into foregut and hindgut

Phylum Rotifera Overview

• 2,000 species

• Can range from 0.1mm-3mm

• Mostly freshwater

• Has a Corona which is a ciliated organ for feeding and locomotion

• Free living

• Filter feeders that are carnivorous

Phylum Nematode Overview

• 16,000 species

• Most abundant animals on Earth

• Can feed on any organic material (soil, water, plants, animals, bacteria, fungi)

• Free living and parasitic

• Can range from microscopic to serval meters long

• Has no cilia (except in sensory structures)

• Their sperm are amoeboid (non flagellated, crawling sperm cells that move using pseudopodia.)

Phylum Rotifera (Head region)

Corona: A ciliated structure that is used for locomotion by creating water currents by beating cilia and aids in feeding by directing food particles toward the mouth.

Mouth: Located behind the corona and functions as an entry point for food particles (plankton, organic debris) swept in by the cilia.

Brain: A small ganglion mass that functions to coordinate sensory input and motor responses (controls cilia and muscle movement)

Flame Bulb: Part of the protonephridia system, it is an excretory structure that is composed of flame cells that helps with osmoregulation (maintaining salt and water balance), and removes metabolic waste.

Phylum Rotifera Anatomy (Trunk Region)

Mastax: A muscular pharyngeal organ containing hard jaw-like structures (trophi) that grinds and crush food before digestion. The mastax can vary in shape depending on the organisms diet.

Trophi: The internal jaws of the mastax, made of chitin. It functions to cut, grind, and puncture food.

Stomach: A digestive sac that chemically digests food using enzymes

Intestines: A narrow tube-like structure extending from the stomach to the cloaca and functions to absorb nutrients from digested food.

Cloacal Bladder (cloaca): A common chamber for waste products from the digestive and excretory system. It collects waste and expels it through the anus.

Germovitellarium: A combination of germarium and vitellarium, germarium produced eggs and vitellarium produces yolk and shell material for the eggs.

Phylum Rotifera Anatomy (Foot region)

Foot: Posterior extension of the body and is often flexible or retractable. It is used for attachment and may have muscular contractions for crawling or gripping.

Anus: expels digestive and excretory waste

Toes: Small extensions at the tip of the foot (usually 2 toes). This helps the rotifer attach to surfaces via pedal glands and assists in gripping or walking.

Pedal glands: Secretes adhesive substances used to temporarily attach to surfaces.

Phylum Rotifera Body Plan

• They are triploblastic, bilateral, and unsegmented

• Do have a coelom but with a coelom not completely lined with mesoderm

• Has a complete digestive tract with regional specialization

• Posterior end has toes with adhesive glands

• Protonephridia

• Males are reduced in number or absent

Phylum Nematoda Body Plan

• They are triploblastic, bilateral, and unsegmented

• Vermiform: worm-shaped, an organism that is elongated, cylindrical, soft bodies, and resembling a worm in shape and movement.

• Considered a coelomate, has a coelom but completely lined in mesoderm

• Body is round in cross section

• Cuticle is elastic

• They have a complete digestive tract with regional specialization

• Excretory system: Renette

• Only longitudinal muscles

Rotifera Organ Systems

Nervous System: Has a brain and paired nerves

Excretory System: Protonephridia (flame bulbs)

No circulatory or respiratory system

Constant cell number approximately 1,000 cells

Rotifera Reproduction

• Asexual: Parthenogenesis where the egg develops into a new individual without fertilization by a male gamete. Eggs are diploid and are developed into genetically identical females. (only females are produced)

• Sexual: Haploid egg and sperm must be fertilized and both male and females can be produced.

• Mix of sexual and asexual reproduction: Based on environmental conditions where amictic and mictic cycles occur. Males and females can be produced.

Rotifera Amictic and Mictic Life Cycles

Amictic: (2n) females produce diploid eggs by mitosis meaning more females are produced. The eggs develop asexually into female clones and no fertilization occurs. This dominates during favorable environmental conditions.

Mictic: Females produce (n) haploid eggs under stress, if left unfertilized the product is a male and if fertilized the thick shelled resting eggs, hatch into amictic (2n) females.

Nematode Body Wall

• Outermost part of the nematode

• Body wall functions al provide shape and protection

  Longitudinal muscles located beneath the hypodermis, that project from the muscles to nerve cord and allows for thrashing movement

• Move via thrashing motion (due to cuticle elasticity and muscle contraction)

• Contains the cuticle

• Also contains a coelom (pseudocoelom) that is filled with fluid under pressure. It acts as a hydrostatic skeleton and proves space for organ development and distribution of nutrient and waste.

• The gut is non muscular and food is primarily moved by pharyngeal pumping. The gut ingests, digests, and absorbs nutrients.

Nematode Organ Systems (Digestive and Excretory)

Digestive System: Mouth → buccal capsule (preps food for passge like cutting and grinding) → muscular pharynx (muscular pump that moves food into the intestine and is basically the esophagus) → intestine → anus

• Digestion is primarily extracellular and occurs in the intestine.

Excretory: The renette cell is specialized for the secretion of waste and helps with osmoregulation. Nitrogen waste (ammonia) is excreted by diffusion across the body wall.

Nematode Reproduction

• Dioecious, often dimorphic (males are usually smaller)

• Reproduce via internal fertilization

• Some have a complex life cycle

• Amoeboid Sperm

Nematode: Ascaris

Infected 800 million people, life cycle includes migration through lungs.

Nematode Organ System (Nervous)

Nervous system: Simple, contains no brain but a nerve ring, sensory & motor & interneurons. Nerve ring acts as a cental processing hub, dorsal nerve cord control motor functions, ventral nerve cord controls both motor and sensory signals, and lateral nerve cord is involved in the excretory system.

Sensory Organs:

• Amphids: Chemoreceptors on the head that detect chemical and odors

• Phasmids: Chemosensory structures in the tail

Nematode: Trichinella spiralis

Acquired by eating undercooked pork

Phylum Mollusca Overview

• 100,000 species (most of the diversity is split into 2 classes, gastropoda and bivalvia)

• Triploblastic

• Bilaterially symmetrical

• Has organ level of organization

• Coelom

• Fresh water, marine, terrestrial

• Protostome development (spiral cleavage, mouth from blastopore, schizocoely)

Phylum Mollusca: Basic Body Plan

Two Part Body: Head-foot (locomotion sensory), and visceral mass (soft, non-muscular part of the mollusks body that houses the internal organs)

Mantle: Secretes the shell and forms the mantle cavity (holds gills and anus)

Radula: Toothed, chitinous tongue (except in bivalves)

Open Circulatory system: Hemocoel

• Bilaterally symmetrical

• Reduced coelom

Phylum Mollusca: Radula

• A toothed tongue like organ used for scraping or cutting food that is found in most mollusks (except bivalves). Composed of chitin and protein.

• It is located inside the buccal cavity (mouth area) and morphology and composition of teeth vary with diet. It continuously produces and moves forward as it wears out, like a flexible conveyor belt.

• The radular ribbon carries the rows of teeth, odontophore is a cartilage-like base that supports and moves the radula, and the radular teeth are arranged in rows.

Phylum Mollusca: Mantle

Specialized tissue layer that covers the visceral mass and forms the outer body wall of a mollusk, as well as secretes the shell. The shell is made of 3 layers, nacreous, prismatic, and periostracum.

• Periostracum (outer layer) is brown or yellow, it protects the mineralized layers from acids, organisms, and abrasion, it also insulates the shell. Produced by the edge of the mantle.

• Prismatic layer (middle layer) made of calcium carbonate, provides most of the shell’s thickness and hardness and acts as a barrier against predators and environmental damage.

• Nacreous Layer (mother of pearl): Iridescent and protects the soft body of the mollusk from the rough interior, it also helps repair damage to the inner shell. It is secreted by the mantle surface throughout the mollusks life.

Phylum Mollusca: Protostome (developmental pattern)

Mollusks are considered protostomes where their mouth forms before their anus. They exhibit a specific pattern of early embryonic development that includes:

• Spiral Cleavage: During early embryonic divisions, cells divide in a spiral pattern. This type of cleavage is determinant meaning the fate of each cell is set early, and it ensures predictable body plans and regulated development.

• Mouth forms from Blastopore: The blastopore (the first opening that forms during gastrulation) becomes the mouth and the anus is formed later from a different region.

• Schizocoelous Coelom: The coelom (body cavity) forms by splitting the mesodermal tissue, and the mesodermal cells migrate and form solid blocks then split to form coelomic cavity.

• Splits in Mesoderm: This is how organs are suspended and how a body cavity develops between the gut and body wall.

• Trochopore Larva: Many marine mollusks have a trochopore larva that is free swimming, ciliated, and looks like a spinning top.

What are the major classes of Mollusca?

Caudofoveata, Aplacophora, Polypalcophora, Monoplacophora, Gastropoda, Cephalopoda, Bivalvia, Scaphopoda

Phylum Mollusca: Aplacophora Overview

• Has 300-400 species

• Usually less than 5mm

• Has a poorly developed head and the shell is typically absent

• Vermiform body (worm body)

• Reduced foot

• Marine

• Bipectinate gills

• Radula may be present

• Monoecious

• Larva and direct development

Phylum Mollusca: Polypalcophora Overview

• Chitons

• Has 800 species

• Reduced Head and Flattened foot

• Eight articulated valves

• All marine rocky substrates

• Uses foot for locomotion and attachment

• Feed mostly on algae

• Rasped with radula (food and surface material is scraped away by the mollusks toothed radula during feeding)

Phylum Mollusca: Polypalcophora Water current intake

Mantle Cavity: Located ventrally (underside) in a narrow groove that runs along both sides of the muscular foot. The cavity lies between the muscular foot and mantle tissue

Gill Arrangements: They have multiple pairs of gills (ctenidia) arranged in the linear series on both sides of the foot within the mantle groove. The number of gills can vary but is often 8 or more pairs matching the 8 dorsal shell plates

Water Flow Directions: Water entered the mantle cavity at the anterior end (front), passes over the gills where gas exchange occurs, then flows posteriorly (toward the end) and exits near the anus. This water flow is crucial for respiration, removal of waste, and gamete release.

Digestive (anus), Excretory (nephridiopores), and Reproductive (gonopores) are openings where water exits.

Phylum Mollusca: Polypalcophora Organ Systems

Digestive System: Complete digestive tract (mouth to anus). Digestion is extracellular meaning it occurs outside cells in the gut lumen, the stomach is the main site for digestion and absorption. They use their radula to scrape algae from rocks.

Nervous System: Possess a ladder-like nervous system that consists of 4 longitudinal nerve cords that are connected by transverse verse (cross connections). The Nerev ring encircles the esophagus and acts like a central processing center. They don’t have a true brain but ganglia act as control centers.

Sensory Structures: Osphradium is the chemoreceptor that detects water quality (near gills). They also have tactile receptors the sense touch and is found in the mantle and foot. The chemoreceptors near the mouth detect food. Lastly, the statocyst is a balance organ located in the foot.

Circulatory System: They have an open circulatory system, a heart that pumps hemolymph (blood-like fluid), and a hemocoel which is the main body cavity through which hemolymph flows.

Gas Exchange System: Gas exchange is performed by the ctenidia (gills) located in the mantle groove, water flows over gills, allowing oxygen uptake and Co2 release.

Excretory System: Uses a pair of nephridia to filter waste from body fluids, they release waste into the mantle cavity for expulsion with water current

Skeletal System: Possess a hydrostatic skeleton that supports body, it is located on the dorsal side where there are usually 8 shell plates that provide protection.

Phylum Mollusca: Polypalcophora Reproduction

Reproductive System: They are dioecious meaning the sexes are separate, fertilization happens externally and gametes are released into the water. Development includes a trochophore larva which is free swimming and ciliated.

Phylum Mollusca: Monoplacophora Overview

• 11 species

• Can be found in the deep sea

• Single symmetrical shell

• Can range from 3mm to 3cm in size

• Possesses a mantle cavity on each side

• Has repeated parts

• 5-6 pairs of monopectinate gills

• Retractor muscles

• Kidneys/Nephridium

• Feed on diatoms, forams, and sponges

Phylum Mollusca: Monoplacophora Organ System

Digestive System: Radula is present and used to scrape food (microorganisms from substrates). Stomach is where their primary digestion occurs and they have a coiled intestine that enhances nutrient absorption and increases surface area. Digestion is extracellular and gut is complete.

Circulatory: They have an open circulatory system where the heart pumps hemolymph through the hemocoel (body cavity). They have multiple auricles and ventricles that are paired.

Gas exchange: Ctenidia (gills) are present in multiple pairs, lining the mantle cavity, water flows over the gills, enabling oxygen uptake and co2 release

Excretory: They have multiple pairs of nephridia (kidney-like organs). The nephridia removes nitrogen wastes (ammonia) and help regulate fluid balance. These also open into the mantle cavity for waste expulsion.

Phylum Mollusca: Monoplacophora Reproduction

• Sexes are separate (dioecious)

• Reproduce externally, fertilization occurs when gametes are released into the surrounding water

• They have paired gonads with one pair on each side, reflecting their segmented body plan.

Phylum Mollusca: Gastropoda Overview

• 35,000 species

• Largest most varied class

• Marine fresh water, terrestrial

• Carnivores, predators, herbivores, detritivores scavengers (feed on dead organic material)

• Well-developed head that is highly cephalized

• Dorsoventral elongation of the body

• Torsion

• Shell morphologies

Phylum Mollusca: Gastropoda (Torsion) VERY IMPORTANT

• Torsion is a feature of gastropod development and anatomy, it is a unique process in the gastropod and alters the body layout of the organism.

• It is a 180 degree counterclockwise rotation of the visceral mass, mantle, and mantle cavity during larval development. The twist occurs in the trochophore or veliger larval stage and results in the mantle cavity, gills, anus, and excretory/reproductive openings moving from the posterior end to a position just behind the head.

• Advantages of torsion includes being able to enter a shell head first which offers protection from predators and environmental threats. Clean water flows into the mantle cavity which prevents covering the gills with waste and improves respiration. The sensory organs are also at the front which allows for improving environmental awareness.

Phylum Mollusca: Gastropoda (Shell Coiling) VERY IMPORTANT

Key evolutionary feature in gastropods that refers to the spiral growth pattern of the gastropod’s shell. It evolved way before torsion meaning early gastropods likely had coiled shells before their bodes twisted 180 degrees.

Coiling evolved to reduce the organisms size, improve balance, and allow animal to withdraw fully into its shell.

Consequences of Coiling: Internal organs become asymmetrical, many species lost paired organs to conserve space (one gill, kidney, etc.), and shell become shield shaped and cone shaped, providing better streamline and balance.

2 Types of Coiling

• Planospiral Shell: coiled in a single plane (garden hose), symmetrical but bulky, seen in some fossil species

• Conispiral (asymmetrical) shell: Coiled in a spiral that tilts upwards, forming a cone shape, this is more compact and efficient, results in asymmetry of the body.

Phylum Mollusca: Gastropoda Body Structure

Head: Prominent and well-developed in most gastropods, equipped with tentacles (often 2 pairs in terrestrial forms), eyes at the tip of tentacles, mouth with a radula, and chemoreceptors for detecting chemicals and food.

Foot: A large muscular ventral structure used for creeping location, while some adapted to swimming, often contains mucous glands for adhesion and movement.

Digestive Tract: Complete gut, begins with a mouth and radula, includes the esophagus, crop and stomach (storage and digestion), intestine often u-shaped (due to torsion), and anus is located near the head in most species (after torsion)

Nervous System: Includes a nerve ring around the esophagus, paired ganglia (cerebral, pedal, pleural, visceral, and buccal). 2 longitudinal nerve cords that are crossed (chiastoneury), controls movement, sensation, and feeding.

Excretory: 1 or 2 nephridia, most have a single left kidney, removes nitrogenous waste (ammonia or uric acid), waste is expelled into the mantle cavity.

Circulatory System: Open circulatory system, heart as 1 ventricle and 1 or 2 auricles depending on gill number. Heart pumps hemolymph (blood-like fluid) into the hemocoel.

Water Flow: Water enters mantle cavity anteriorly then passes over the ctenidia for gas exchange. Water exits posteriorly or laterally to flush out excretory waste, reproductive gametes, and digestive waste.

Phylum Mollusca: Gastropoda Locomotion

They move using a specialized flattened foot located on the ventral side of the body.

• Flattened Foot: Broad and muscular, used for creeping

• Cilia: Microscopic hair like structures on the foot aid in gliding.

• Gland Cells: Secrete mucus, which reduces friction and allows smother movement

• Muscular Contractions: Create wave-like motions (pedal waves) to propel the animal forward.

Phylum Mollusca: Gastropoda Feeding

Most are herbivores or detritivores using a radula to scrape food.

Radula: A ribbon like structure with rows of tiny chitinous teeth

Scraping action: Used to rasp algae or detritus off surfaces

Extensible Proboscis: The mouth is on an extendable tube (proboscis) that is used to reach prey and deliver radula or toxins

• Some species evolved radula adapted for piercing or cutting prey, for example in cone snails the radula is modified into a single harpoon like tooth. The tooth injects venom (a neurotoxin) into the prey like small fish or worms. The toxin can paralyze or kill prey.

Phylum Mollusca: Gastropoda (Digestive Tract)

They have a complete and specialized digestive system that processes food through both extra cellular and intracellular digestion by cilia, mucus, and digestive glands.

The digestive system is ciliated, helping to move food along the tract, the food is then trapped in music forming a mucoid mass.

The mucoid mass combines with a Protostyle (rotating mucus rod in the stomach) that helps mix and grind food as it rotates, aiding in digestion.

The digestive gland located in the visceral mass secretes acid and digestive enzymes where they enter the stomach to break down food.

2 types of digestion: Extracellular (happens in the stomach, enzymes and acids digest food outside the cells) and Intracellaular (occurs in the digestive gland where nutrients are absorbed into cells where final digestion takes place)

Waste formation: In the intestine, undigested material is compacted into waste pellets that are eventually expelled.

Phylum Mollusca: Gastropoda Gas Exchange and Circulatory

Ctenidia (gills) are the primary respiratory structure that can be paired or single, depending on the species. Found in the mantle cavity and water flows over the gills allowing for oxygen uptake and co2 release.

They possess an open circulatory system, blood flows through blood sinuses (open cavities) that bathe tissues directly. Functions to circulate nutrients and oxygen, also acts as a hydrostatic fluid, helping to support body structure and movement.

Phylum Mollusca: Gastropoda Nervous System

• Larger head ganglia, well developed cerebral ganglia (brain like clusters of nerve cells in the head) Coordinate sensory input and motor control

• Nerve ring: surrounds the esophagus and connects major ganglia (cerebral, pleural, pedal)

• Two nerve cords: extend from the nerve ring through the body, can become crossed.

• Eyes: Simple light detection

• Statocyst: Balance and orientation

• Osphradia: Located near the gills; used for chemoreception (detecting chemicals in water)

Phylum Mollusca: Gastropoda Excretory System

Nephridium: The primary excretory organ in gastropods. Sac-like structure connected to the coelom. Collects and filters waste products from the coelomic fluid. The waste is then secreted into the blood, and further modified before being excreted.

• Aquatic Gastropods

  • Ammonia is the primary waste excreted. Ammonia is highly soluble in water, so it can be efficiently released into the surrounding environment.

• Terrestrial Gastropods

  • Uric acid is the primary waste excreted. Uric acid is more energy-intensive to produce but is water-conserving, which is important for land-dwelling species to avoid dehydration.

Phylum Mollusca: Bivalvia Overview

• 30,000 species

• Clams, Oysters, Mussels

• Has 2 valves (shells

• Sheet-like mantle

• Laterally compressed

• Mostly Filter feeders

• No head or radula

• Attached or sedentary

Phylum Mollusca: Bivalvia Shell

Two parts (valve): the bivalve shell consists of 2 hinged valves which are symmetrical and typically made of calcium carbonate.

Proteinaceous hinge: The hinge that connects the two valves is made of protein and is located dorsally (on the top).

• Elastic: The hinge is elastic, allowing the valves to open and close.

• Teeth and projections: The teeth or projections on the hinge help align and secure the two valves together. This ensures the shell stays properly closed.

• Adductor muscles:

  • Two pairs of adductor muscles are responsible for closing the shell. These muscles are very strong and allow the bivalve to tightly close its shell to protect itself from predators or environmental threats.

• Mantle: The mantle (a soft tissue layer) is attached to the shell margin, surrounding the adductor muscles.

  • The mantle is involved in secreting the shell material and plays a role in the animal's respiratory and excretory systems.

• Oldest part (umbo):

  • The umbo is the oldest part of the shell, located at the anterior (front) region. It is the first part of the shell to form during development.

Phylum Mollusca: Bivalvia General Structures

Foot: The foot in bivalves is a muscular structure used for locomotion and anchoring the animal to its substrate. In burrowing species, it helps dig into the sediment and allows the bivalve to anchor itself securely

Visceral Mass: The visceral mass contains most of the bivalve's internal organs including the digestive, excretory, and reproductive systems. This mass is located under the mantle and is not directly involved in shell formation.

Gill: Bivalves have gills for both gas exchange and feeding

Organ Systems

• Digestive System: Bivalves have a complete digestive tract, with a mouth and anus. They filter food from water, using their gills for both feeding and gas exchange.

• Excretory System: Nephridia (excretory organs) remove nitrogenous waste and excess water. In aquatic species, ammonia is excreted, while in terrestrial species, uric acid is typically produced.

• Reproductive System: Most bivalves are dioecious, with separate sexes. External fertilization is common in many species, releasing eggs and sperm into the water. Some species also exhibit external larval development, such as trochophore and veliger stages.

Phylum Mollusca: Bivalvia Gills

Types of Gills in Bivalves

1. Protobranch Gills: Primitive type. Primarily used for respiration only, not feeding. Found in deep-sea and basal bivalves.

2. Lamellibranch Gills: Advanced, complex and folded. Used for both gas exchange and filter feeding. Common in most modern bivalves.

3. Dorsal of gills: exhalant and Ventral Gills: Inhalant

   Gills are ciliated and move water through the mantle cavity, mantle in some bivalves is extended into incurrent and excurrent siphons.

Phylum Mollusca: Bivalvia Gas Exchange

• Water Flow: enters through anterior end and exits posterior

• Gills are made of filaments (lamella) where water can pass through tubes

• Blood vessels are close to passing water but circulating fluid and water never mix

• Highly efficient gas exchange system:

  • Water and blood flow in opposite directions (countercurrent).

  • Ensures maximum oxygen uptake because:

    • Low O₂ blood always meets low O₂ water,

    • High O₂ blood always meets high O₂ water,

    • Maintaining a favorable gradient along the entire gill surface.

• Blood Vessels:

  • Afferent vessels: Bring deoxygenated blood out the gills.

  • Efferent vessels: Carry oxygenated blood into gills.

Phylum Mollusca: Bivalvia Feeding

Food is captured on the gill: Food particles from water enter though incurrent siphon where cilia gills trap and collect the food particles

Cilia move it to the margins of the gill from there the cilia then moves it to the mouth

Labial Palps: flaplike structures that sort the food and direct acceptable food to the mouth, unwanted particles go to sorting area

Rejected Material (nonnutritive particles) is not swallowed but is packed into psuedofeces and expelled from the shell without entering the digestive system.

Some clams are in symbiotic relationships with algae, algae perform photosynthesis, producing nutrients that benefit the bivalve and in return the clam provides a safe light accessible environment.

Phylum Mollusca: Bivalvia Digestion System

1. Mouth: Located between the labial palps. Entry point of food into the digestive tract.

2. Esophagus: Short tube that transports food from the mouth to the stomach.

3. Style Sac: Muscular chamber where the crystalline style is located. The sac rotates, grinding food.

4. Crystalline Style: A rod-like structure made of mucus and enzymes. Rotates against the gastric shield, grinding and releasing digestive enzymes into the stomach.

5. Gastric Shield: A tough pad in the stomach wall. Protects the stomach from damage by the rotating style.

6. Stomach: Site of extracellular digestion. Food mixed with enzymes and mucus to break down larger particles.

7. Digestive Gland (Hepatopancreas): Secretes enzymes. Site of intracellular digestion and nutrient absorption. Functions similar to a liver and pancreas combined.

8. Intestine: Absorbs water and nutrients. Forms fecal pellets from undigested material.

9. Anus: Located near the excurrent siphon. Expels waste out of the body, into the surrounding water.

Phylum Mollusca: Bivalvia Nervous System

Three pair of interconnected ganglion associated with esophagus ganglion (controls basic digestive functions and located around the esophagus), foot ganglion (associated with movement), posterior adductor (control adductor muscles)

Mantle primary sensory organs: help detect environmental changes

Tentacles: sensitive to touch and pressure

Simple or complex eyes: Photoreceptors that can detect light and dark, but have poor image forming abilities

Osphradium: Chemosensory helps sense chemical signals in the water

Statocyst: Balance and Position

Phylum Mollusca: Bivalvia Circulatory System

Heart: located in the pericardial cavity, multichambered, has one ventricle and 2 atria that pumps hemolymph through the open circulatory system

Arteries: Heart pumps hemolymph into the arteries which carry the fluid to various parts of the body

Veins: Return hemolymph from tissues back to the heart via atria before being pumped out through the ventricle again.

Blood sinuses in foot: Open spaces where hemolymph circulates, foot contains blood sinuses helping with movement and control of the foot during locomotion and burrowing.

Phylum Mollusca: Bivalvia Reproduction

• Majority are dioecious but some species are monoecious and can produce via cross fertilization

• Gonads in Visceral Mass: Gonads are located in the visceral mass, the central part of the bivalve’s body that contains most of its internal organs. Gonads are connected to the mantle cavity through gonoducts where they release gametes into the water.

• External Fertilization: Sperm and egg released in water meet up in the water column

Phylum Mollusca: Cephalopoda Overview

• 600 species

• Octopus and Squid

• Most complex Mollusk

• Anterior foot modified in tentacles (prey capture, attachment, copulation)

• Size ranged from a few cm to 16 meters or more

• Foot modified into siphon jet propulsion

• Body enclosed by muscular mantle

• All have ink glands

Phylum Mollusca: Cephalopoda Shell

Nautilus: The only cephalopod with a physical shell. The shell is spirally coiled and divided into chambers. The nautilus lives in the last chamber (largest one) while the other chambers are filled with gas for buoyancy control. A cord of tissues (siphuncles) are in the unoccupied chambers and help regulate gas levels in each chamber.

• Shell is reduced or absent in most cephalopods, evolutionary change allowed for greater mobility and flexibility.

• Cuttlefish has in internal calcareous shell that helps with buoyancy but does not offer external protection.

• Squid (pen): Squids have a reduced internal shell called a pen that is thin and flexible. It is a remnant of the cephalopod shell and is used for support and maintaining body shape.