BIOL 112 Lecture 25
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Animals
Muscles
- Muscle is made of bundles of muscle fibers
- Each fiber is made from several cells containing myofibril
- Myofibril is segmented into sarcomere units made of actin and myosin filaments
Nervous System
Animal taxa differ in circuit formations, targets (muscle, etc), and patterns of neurons arrangements:
- cnidarians: neural net
- platyhelminthes: dual nerve cords and nerve cells clumped in ganglia
- arthropods: multiple segmental ganglia
- mollusc: cephalic brain
- chordate: cephalic brain, single spinal cord
Coelom
Diffusion is sufficient for tissue only a few cells thick. As tissues get thicker, diffusion becomes less efficient. This limits organism size.
Animals overcame this problem with a fluid-filled cavity called a coelom.
- acoelomates
- platyhelminthes (flatworms)
- no body cavity
- pseudocoelomates
- nematoda (roundworms)
- coelom partially lined in with mesoderm
- fluid called pseudocoelomic fluid
- coelomate
- annelida
- true coelom with mesoderm on both faces
- fluid called coelomic fluid
Origins of Animals
Colonial flagellated protist ancestor: choanoflagellates
colony of identical cells → functional specialization of cells → interdependence of cells → multicellular animal
Some extant animals have choanoflagellate-like cells:
- sponges
- flatworms
- cnidaria
Cambrian explosion (~550 mya) was the introduction of most animal phyla, including first vertebrates