Animal Diversity II

Cell Development and Fate

• Determinate vs Indeterminate Development

  • Determinate Development: Cell fate is determined early in development.

  • Indeterminate Development: Cell fate is determined later, allowing for more flexibility in developmental outcomes.

• Experimental Studies in Development

  • Researchers often conduct experiments where they remove or reposition cells in developing embryos (e.g., in frogs and chickens) to observe impacts on growth and differentiation.

  • Questions include:

    • Does removing a cell halt development?

    • Can a cell survive if moved to a different location?

    • Will the cell retain its original fate or take on a new role due to the surrounding signaling environment?

Embryonic Development Observations

• Coel Development:

  • Cells move apart during development to form the mouth and other structures.

  • In deuterostomes, cell layers form on top of one another, affecting fate determination.

• Challenges in Developmental Studies:

  • An example discussed involved manipulating a gene in mice to study signaling, where the expression timing was crucial.

  • New findings can lead to unexpected outcomes, such as brain anomalies instead of cancer.

Clades of Protostomes

• Spiralia

  • Embryos develop using spiral cleavage

  • Most live in water

  • Move using cilia or contractions of the body musculature

  • Two clades

    • Platyzoans

      • flatworm - no circulatory (use gas diffusion) or respiratory systems

    • Lophotrochozoans

      • Phylum Mollusca

      • Trochophore: a free-living larva

      • Lophophore: a horseshoe-shaped crown of ciliated tentacles surrounds the mouth used in filter-feeding

• Ecdysozoa

Excretion + Osmoregulation

• Network of fine tubules runs through body

• Flame cells located on the side branches

  • Flagella move water and excretory substances into the tubules and then to pores located between the epidermal cells through which the liquid is expelled

• Metabolic wastes are excreted into the gut and eliminated through the mouth

Nervous System and Reproduction

• Simple nervous system

  • Anterior cerebral ganglion and nerve cords

  • Eyespot can distinguish light from dark

• Reproduction

  • hermaphroditic

  • sexual reproduction

  • asexual regeneration

Rotifer

• Platyzoans

• Bilaterally symmetrical, unsegmented pseudocoelomates

• Highly developed internal organs

• Corona – “wheel animals”

  • Conspicuous ring of cilia at anterior end

  • Used for locomotion and sweeping food into the mouth

Characteristics of Animals

• Animal Segmentation:

  • Repeated patterns observable in evolution, helping characterize different species.

  • Some organisms show a decentralized nervous system.

• Unique Features in Eumetazoans:

  • Nematocysts used for prey capture are noted characteristics of specific phyla, demonstrating their ecological role.

Gastrovascular Cavity Structure

• Observations:

  • Single opening for intake and digestion, leading to a short gastrovascular cavity.

  • Example: Capture of prey such as brine shrimp via paralyzing toxins.

Evolutionary Branching in Animals

• Understanding how animals branch off evolutionary trees is essential for studying biodiversity and ecosystem dynamics.

• Discussion of invasive species and their impact on local ecology was highlighted, emphasizing the importance of environmental monitoring.

Lophotrochozoa (Mollusks) and their Anatomy

• Clams,snails,slugs,octopuses

  Mantle

  • thick epidermal sheet

  • bounds mantle cavity

  • secrete shell

  Foot

  • primary means of locomotion

  • divided into arms or tentacles in cephalopods

  Internal Organs

  • Coelom is reduced

  • digestive, excretory, and reproductive organs are concentrated in a visceral mass

  • ctenidia - gills in aquatic mollusks

  • Circulatory system - open in most mollusks, allowing blood to flow freely through the body cavity.

Mollusk Body Plan

Shell

• Protects against predators and adverse environments

• Secreted by outer surface of mantle

• Clearly not essential – repeated loss or reduction

• Typical shell has 2 layers of calcium carbonate

  • Internal layer may be mother-of-pearl or nacre

  • Pearls are formed by coating foreign object with nacre to reduce irritation

Radula

• Rasping, tonguelike structure used in feeding

• Used to scrape up algae

• Bivalves do not have a radula

  • Gills used in filter feeding

Waste Removal and Circulatory System

• Nitrogenous waste removal – nephridia

  • Consist of cilia-lined openings called nephrostomes

  • Tube to excretory pore to mantle cavity

• Circulatory system

  • Open circulatory system

    • Hemolymph sloshes around hemocoel

    • 3-chambered heart

  • Cephalopods have a closed circulatory system

Mollusk Reproduction

• Most mollusks are gonochoric (individuals are either male or female)

  • A few are hermaphroditic

  • Some oysters change sex

• Most engage in external fertilization

  • Gastropods have internal fertilization

• Mollusk zygote undergoes spiral cleavage

Classes of Mollusks

1. Polyplacophora - Chitons

2. Gastropoda - limpets, snails, slugs

3. Bivalvia - clams, oysters, scallops

4. Cephalopoda - squids, octopuses

Bivalvia

• Includes clams, scallops, mussels, oysters, and others

• Most marine, some freshwater

• No radula or distinct head

• Have 2 shells (valves) hinged together

  • Adductor muscles counter hinge ligament

• Water enters through inhalant siphon and exits through exhalant siphon

  

Cephalopoda

• Only mollusk with closed circulatory system

• Foot has evolved into a series of arms equipped with suction cups

  • Beak-like jaws, toxic saliva

• Characteristics of cephalopods differ significantly from bivalves; they have complex behaviors without an external shell.

• Functional aspects were demonstrated through dissections, emphasizing anatomical adaptations for mobility and feeding.

Siphons and skin

• cephalopods lack external shell

• Jet propulsion using siphon

• ink can be ejected from siphon

• Chromatophores allow for changing skin color for camouflage or communication

Lophotrochozoans: Phylum Annelida

• Segmented worms

• Body built of repeated units

Body plan

• Head has well-developed cerebral ganglion

• Sensory organs in ringlike segments

• Many species have eyes

• Segments divided internally by septa

  • Each segment has a pair of excretory organs, a ganglion, and locomotory structure

• Closed circulatory system

• Ventral nerve cord

• Locomotion

  • Coelomic fluid creates a hydrostatic skeleton

  • Alternating muscle contractions allows complex movements

  • Chaetae – bristles of chitin found in most groups

• Closed circulatory system

  • Gas exchange by diffusion across body surfaces

• Excretory system – nephridia similar to mollusks

Arthropods and Their Importance

• Major contributors to ecosystems with over 2,000,200 species.

• Key Features:

  • Jointed appendages, including antennae and mouth parts, enhance survival.

  • Omnidirectional vision is achieved through the compound eyes, made of repeated units (ommatidia) that connect to the nervous system for processing visual information.