Invertebrates #3

What is the significance of Cnidarians and Ctenophora being diploblastic?

Cnidarians and Ctenophora are diploblastic, meaning they develop from two germ layers—ectoderm and endoderm.
🔹 This allows them to form true tissues (like nerve and muscle tissues), which Porifera cannot do.
🔹 It enables radial symmetry, better body organization, and the development of a simple nerve and digestive system (gastrovascular cavity).
🔹 It marks a major evolutionary step toward more complex animals.

Jellyfish lifecycle stages (Cnidarians)

1. polyp stage, which is sessile and asexual

2. medusa stage, which is free-swimming and sexual.

What do nematocytes do in cnidarians?

Nematocytes are stinging cells that contain a capsule called a nematocyst. They shoot out a thread to inject toxins, helping cnidarians catch prey and defend themselves.

What are the characteristics of the polyp body form in cnidarians?

• Diploblastic: has ectoderm (outer) and endoderm (inner) layers

• Radial symmetry

• Sessile (attached) lifestyle

What kind of nervous system do cnidarians have?

-simple nerve net with no brain or central nervous system. It allows them to detect stimuli and coordinate movements like tentacle contractions.

How do cnidarians obtain energy?

• Carnivorous: inject prey with toxins and capture it

• Digestive method: extracellular digestion in the gastrovascular cavity

• Symbiosis: corals and anemones get extra energy from symbiotic algae (dinoflagellates)

Why did early cnidarians evolve nematocysts if there were only simple animals like sponges around?

• Defense against predators or competitors

• Capturing small prey like plankton or protozoans

• Competing for space on the ocean floor
  They didn’t need large animals to hunt—any advantage helped them survive.

Did cnidarians have nematocysts when they first diverged from other animals?

Early cnidarians likely had a simple body plan. Nematocysts and other specialized features developed later as the clade evolved for feeding, defense, and competition.

What are the key characteristics of Hydrozoa?

• Mostly colonial or solitary cnidarians

• Can have both polyp and medusa stages, with polyp often dominant

• Include freshwater and marine species (e.g., Hydra, Obelia)

• Use cnidocytes for capturing prey and defense

What does it mean when a cnidarian is solitary or colonial?

• Solitary: Lives alone and does everything itself (e.g., Hydra)

• Colonial: Many connected individuals share resources and may specialize in tasks like feeding, reproduction, or defense (e.g., Obelia)

What are the key characteristics of Scyphozoa?

• True jellyfish; mostly medusa-dominant

• Polyp stage is small and short-lived

What are the key characteristics of Cubozoa?

Also called box jellyfish; mostly medusa-dominant

What are the key characteristics of Anthozoa?

• Includes corals and sea anemones

• Polyp-only life cycle; no medusa stage

• Sessile (attached) and often colonial

• form mutualistic relationships 

What is coral bleaching and why is it important?

Coral bleaching happens when corals expel their zooxanthellae due to stress (like warm water, UV radiation, pollution, or disease), causing them to lose color and energy and risking coral death.

What key traits caused the next branch leading to Platyhelminthes on the animal phylogeny?

branch point where animals first evolved triploblastic body layers (3 germ layers) + bilateral symmetry.

Why do Platyhelminthes have a simple nervous system while Porifera and Cnidaria do not?

• Porifera: no true tissues → no nervous system

• Cnidaria: nerve net only, no brain or central system

• Platyhelminthes: bilateral symmetry and cephalization → simple nervous system with head ganglia and longitudinal nerve cords for directed movement and sensing

What are Trematoda and their characteristics?

• Trematoda = flukes, endoparasitic flatworms

• Live inside hosts (endoparasites)

• Often have a complex life cycle with a primary host (sexual reproduction) and an intermediate host (transitional stage, e.g., cercaria)

• Example: schistosomes cause schistosomiasis

How is cephalization connected to bilateral symmetry?

• Bilateral symmetry gives animals a front (head) and back (tail) for directed movement

• Cephalization is the concentration of sense organs and nervous tissue in the head

• It allows animals to detect stimuli and respond efficiently while moving forward

What is a coelom?

A coelom is a fluid-filled body cavity completely lined with mesoderm that allows space for organ development and movement.

Which invertebrate group is the first to have a true coelom?

Annelida (segmented worms)

What is the difference between Protostomes and Deuterostomes in terms of blastopore (first opening) development?

Protostome:
• “Mouth first” — blastopore becomes the mouth
• Examples: Annelida, Mollusca, Arthropoda

Deuterostome:
• “Anus first” — blastopore becomes the anus
• Examples: Echinodermata, Chordata (humans)

Radial Cleavage

Radial: cells divide at angles

Determinate cleavage = each cell’s fate is already decided 

• Helps build structured body plans 

Spiral Cleavage

Spiral: cells divide right on top

Intermediate cleavage = cells can become any type 

• good for complex organ systems/regeneration 

Why can deutrstomes (like humans) form twins and stem cells

Because of Radial cleavage allows flexible development where cells can change rolls

What is a trochophore larva, and in which groups is it found?

A trochophore larva is a free-swimming (planktonic) larval stage used for dispersal in aquatic environments, and it is commonly found in marine Annelida and Mollusca.

What is metamerism and why is it important in Annelids?

• Metamerism = linear repetition of similar body segments (metamers)

• Allows flexible movement, efficient growth, and specialized segment functions

• Improves coordination and nervous control

What are key physical and structural features of Annelida (segmented/ringed worms)?

• Body is divided into metamers (segments)

• Found in aquatic and moist terrestrial environments

• Have a coelom, used as a hydrostatic skeleton

• Segmentation allows efficient nervous control and movement

Why is metamerism (segmentation) advantageous in Annelids?

1. Multiple copies of organs and structures 

2. Efficient nervous control 

3. Body size can increase easily 

4. Regeneration ability 

What major body systems do Annelids have?

• Circulatory system: Closed circulation (blood stays in vessels)

• Digestive system: Complete (mouth → anus)

• Nervous system: Ventral nerve cord with ganglia in each segment

• Excretory system: Nephridia in most segments

• Respiratory system: Gas exchange through skin (cutaneous respiration)

• Skeletal system: No hard skeleton — uses coelom as hydrostatic skeleton

• Reproductive system: Can be sexual or asexual (some hermaphroditic)

What are key features of Polychaeta (bristle worms)?

• Marine annelids, includes bristle worms and fan worms

• Parapodia+: muscular flaps with setae (bristles)
   → Used for locomotion and respiration (gas exchange)

• Detritivores or filter feeders

What are the key features of Oligochaeta (earthworms)?

• Mostly terrestrial, some freshwater (aquatic)

• Have light-sensitive cells, but no true eyes

• Detritivores – help recycle nutrients in soil

• Hermaphroditic – have both male and female reproductive organs

• Clitellum+ – secretes a cocoon for embryo development

• No specialized larval stage (direct development)

Hirudina (leeches)

• Mainly freshwater 

• Ectoparasitic and carnivorous 

• Anterioir + posterior suckers 

• Hermaphroditic 
  

What links the Annelida + Mollusca

Trochophore larva:

swimming, ciliated, marine baby stage for annelids and molluscs; used for feeding, movement, and dispersal.

Mollusca (sister group to Annelida)

terrestrial or aquatic

• trochophore larva

• billateral symmetric

• triploblastic

• protostomes with reduced coelom

Unique traits of Mollusca

Mantle

• Dorsal sheet of tissue

• Secretes a shell or tiny spicules

• Forms the mantle cavity containing gills

• Mantle cavity connects to the sea for gas exchange (breathing)

Radula

• Rasping, tongue-like organ

• Used for scraping and ingesting food. 

Foot

• Ventral muscular organ

• Used for locomotion, creeping, burrowing, or swimming

Mollusca Body plan

Bivalvia - mucles

Polyplacophora - chitons

Gastropoda - snails/slugs

Cephalopoda - squid/octopus

Polyplacophora (chitons)

• 8 overlapping shell plates

• Large muscular foot (strong grip)

• Radula scrapes algae (herbivore)

• Gills in mantle grooves

• Ability to roll into ball 

Bivalvia (mussels)

• Two-hinged shells, no distinct head

• Large muscular foot (digging)

• Mantle forms siphons for water flow

• Enlarged gills for breathing & filter feeding

• Siphons draw water across gills to trap food

Scallops’ eyes and large adductor muscles are examples of what type of adaptation?

structural adaptations to the environment, showing how body features evolved for mobility and predator detection — helping scallops survive in open, predator-rich habitats.

Gastropoda (snails/slugs)

• Largest mollusk group (most species)

• Terrestrial or aquatic (conquered land gill→ lung)

• Herbivores, predators, or scavengers

• Can be shelled (snails) or shell-less (slugs, nudibranchs)

• Complex head with eyes and tentacles

• Torsion

Special Modification – Torsion in Gastropods

• 180° twist of body during development

• Moves mantle cavity & anus over the head

• Helps with protection (can pull head into shell first)

Example of radula modification for predation in Mollusca?

Cone Snails (Genus Conus)

• Radula modified into a harpoon-like tooth

• Injects venom to paralyze prey

• Used for active predation (fish, worms, other mollusks)

• Example of evolutionary adaptation of radula for predation

How do Nudibranchs (sea slugs) defend themselves, and what makes them unique?

Nudibranchs (“naked gill”)

• Bright, colorful bodies = warning coloration (aposematism)

• Contain toxins or stinging cells for defense

• Some eat cnidarians (jellyfish, anemones) and steal their cnidocytes

• Store them in their own tissues for protection

• Example of defensive adaptation and mimicry

What are the key features and major groups of Cephalopoda (squid, octopus, cuttlefish, nautilus)?

• Foot subdivided → arms & tentacles

• Enlarged head, reduced or lost shell (except Nautilus)

• Arms/tentacles with suckers/hooks – active predators

• Excellent vision, complex behavior

• Visual communication using color & texture

• Masters of camouflage & inking

What is shell reduction in Cephalopods, and why is it important?

Shell Reduction in Cephalopods

• Ancestral mollusks had external shells

• In cephalopods, the shell became internal or lost

• Nautilus = external shell (primitive)

• Squid & cuttlefish = internal shell (pen, cuttlebone)

• Octopus = shell completely lostAdvantages:
  ✔ Greater mobility & speed
  ✔ Better camouflage & flexibility
  ✔ Easier burrowing & hiding
  ✔ Supports complex predatory lifestyle

Cephalopods traits

• Predators with excellent vision (complex behavior) 

• Visual communication with color + texture  

• Masters of disguise 

• Inking

What are the key characteristics of Nematoda (roundworms) in Ecdysozoa?

Nematoda (Roundworms)

• Found in all habitats (soil, water, inside hosts)

• Feed on detritus, bacteria, fungi, or live as parasites

• Unsegmented, elongated, cylindrical, limbless body

• Pseudocoelomate, with tough cuticle → must molt (Ecdysozoa trait)

Arthropoda characteristic features 

1. Segmented body plan

2. Jointed appendages 

3. Rigid exoskeleton 

4. High degree of cephalization with elaborate sensory oragns 

Colemn is reduced (no hydrostatic skeleton)

Developed gills/lungs/tracheae

Segmented body plan (Arthropoda)

• Segments are specialized and fused to form *tagmata 

• *specialized grouping of multiple segments) 

• Head, thorax, abdomen (distinct body regions in arthropods)