Animal Diversity II

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?

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
- animals that molt
- exoskeleton
  - hard external shell
- 2 groups
  - arthropods
  - nematodes

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

Simple nervous system
- Anterior cerebral ganglion and nerve cords
- Eyespot can distinguish light from dark
Reproduction
- hermaphroditic
- sexual reproduction
- asexual regeneration

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

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.

Observations:
- Single opening for intake and digestion, leading to a short gastrovascular cavity.
- Example: Capture of prey such as brine shrimp via paralyzing toxins.

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.

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

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

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

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

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.

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

Segmentation
- In some classes specialized into tagmata
- Head, thorax, abdomen
- Head and thorax may be fused into cephalothorax or prosoma
Exoskeleton
- Made of chitin and protein
- Protects against water loss
- Must undergo ecdysis – molting
Jointed appendages
- May be modified into antennae, mouthparts, or wings
- Can be extended and retracted

Open circulatory system
Nervous system
- double chain of segmented ganglia
- ventral ganglia control most activity
Compound eyes are found in many arthropods
- Composed of independent visual units called ommatidia
- Other arthropods have simple eyes, or ocelli
- May be in addition to compound eyes
- Have single lenses
- Distinguish light from darkness