Biology Chapter 32

Study Notes

Multicellular diploid stage is the dominant stage in animals' life cycles
Cleavage: a series of rapid cell divisions without growth that converts the zygote into a ball of cells
Stages of development:
- Blastula: a hollow ball of cells
- Gastrula: a stage where the blastula undergoes invagination to form layers of embryonic tissues (ectoderm, mesoderm, endoderm)

Animals are most closely related to choanoflagellates
Ediacaran biota: ancient, mostly soft-bodied organisms from the late Precambrian
Three hypotheses to explain the rise of Cambrian diversity and the decline of the Ediacaran biota:
- Predator-prey relationships: evolution of new predator species leading to diverse adaptations
- Increase in atmospheric oxygen: allowed larger and more metabolically active animals to thrive
- Evolution of Hox genes: led to developmental flexibility and new body plans

Two types of symmetry found in animals: radial symmetry and bilateral symmetry
Germ tissue layers present in animals: ectoderm, mesoderm, and endoderm
Organs associated with each germ layer:
- Ectoderm: skin, nervous system
- Mesoderm: muscles, bones, circulatory system
- Endoderm: digestive tract, respiratory system
Diploblastic animals have two germ layers (ectoderm and endoderm), while triploblastic animals have three germ layers (ectoderm, mesoderm, and endoderm)
Coelom: a body cavity entirely within the mesoderm
Hemocoel: a primary body cavity containing hemolymph (circulatory fluid in organisms with an open circulatory system)
Acoelomates: animals without a coelom
Molluscs have both a coelom and a hemocoel

Two body plans: polyp and medusa
Gastrovascular cavity: a digestive system with a single opening (mouth) that serves as both entrance and exit for food and waste

Most have bilateral symmetry
Lophophore: a crown of ciliated tentacles used for feeding
Platyhelminthes (Flatworms): flat bodies increase surface area relative to volume, facilitating diffusion of gas, waste, and nutrients
Rotifers: have a form of asexual reproduction called parthenogenesis, where females produce offspring from unfertilized eggs

Tough external coat called a cuticle
Process of shedding the cuticle called ecdysis
Nematodes: unsegmented body, pseudocoelom, and a complete digestive system
Arthropods: segmented body, exoskeleton, and jointed appendages
Exoskeleton provides protection, support, and prevents water loss
Open circulatory system: circulatory fluid (hemolymph) bathes the body cavity
Three major lineages of Arthropods: Chelicerates, Myriapods, and Pancrustaceans
Myriapods: Centipedes (one pair of legs per segment, carnivorous) and Millipedes (two pairs of legs per segment, detritivorous)
Insects: development of wings, diversification of mouthparts, and coevolution with plants led to an explosion in evolution
Metamorphosis: insects undergo either incomplete metamorphosis (nymph stages resemble adults) or complete metamorphosis (larval stages do not resemble adults and go through a pupal stage)