BIOL 211 Lecture 27: Animals Part 3

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California State University - Long Beach
BIOL 211
Bruno Pernet

Lecture 27 Notes: Animals Part 3 Animals: Bilaterians • All triploblasts • Origin: molecular data suggests the bilaterian common ancestor lived ~670 mya, but we don’t have good fossils until much later o Most bilaterian phyla appeared in the fossil record in the Cambrian explosion • Broken up into three big clades; Lophotrochozoa + Ecdysozoa clade is often called the “protostomes” Animals: Lophotrochozoans • Huge clade: ~18 phyla o Just identified as a clade ~20 years ago on the basis of molecular data o Most members of the clade have a trochophore larva (indirect life cycle) or a lophophore (but some have neither) ▪ Trochophore larva: a tiny, planktonic larval stage found in some LT (molluscs, annelids); have a band of cilia for locomotion and feeding ▪ Lophophore: a ring of ciliated, hollow tentacles used for feeding and gas exchange in some LT (ectoprocts and brachiopods) o Very, very diverse • Platyhelminthes: flatworms, ~20,000 species o All are unitary and acoelomates (mesoderm fills all space between ectoderm and endoderm) o Most are aquatic, both freshwater and marine, but some are terrestrial in damp places o No special organs for gas exchange or circulation, so to get O 2o all cells, they need a high SA/volume ratio  flat o Free-living flatworms are usually predators or scavengers; some are parasites o All have incomplete guts (a mouth but no anus) except tapeworms, which don’t have any gut at all o Best known are “planarians,” who live in ponds and streams ▪ Scavengers or predators; capture food at the mouth and digest it ▪ Concentration of nerve cells and sensory structures (eyespots) at anterior end ▪ Able to regenerate if cut into pieces; some can reproduce asexually by breaking into pieces o Parasitic flatworms: ▪ Trematodes (flukes): internal parasites of vertebrates that eat host tissue; complex life cycle with two different largal stages and two different hosts; schistosomiasis infects ~200 million people worldwide and can permanently damage liver, spleen, bladder, lungs ▪ Tapeworms: internal parasites of vertebrates that live in host gut, attached by the spines and suckers of the scolex and absorb food across the tapeworm body wall; acoelomates with no mouth or gastrovascular cavity; body is a chain of proglottids that covers ovaries and testes; often have complex life cycles with 2+ hosts • Syndermata: ~2900 species o Most are rotifers: tiny, unitary, and pseudocoelomates (some mesoderm, but not completely lined with it; mostly fluid-filled cavity) o Corona – ring of cilia at the anterior end that is used to swim and feed o Most are suspension feeders, some are predators o Mastax – structure that contains jaws used to chew food o Most are aquatic (freshwater and marine), some wet terrain ▪ When dry, rotifers lose almost all their body water and enter an inactive, “cryptobiotic” stage in which metabolism stops completely ▪ They can stay this way for decades o Most rotifers undergo cyclical parthogenesis: females make diploid (!) eggs that can develop without fertilization ▪ They can reproduce asexually when conditions are good, and sexually when conditions are bad (i.e. low temperature, limited food) ▪ Sexual: zygote is produced that can survive through harsh conditions o Some rotifers never have sex (Bdelloids are all female) • Ectoprocts: ~4500 species o All colonial and coelomates (fluid-filled cavities called coeloms, completely lined by mesoderm) o All use a lophophore for suspension feeding and gas exchange o Colonies are composed of many tiny individuals, each living in a box of skeleton it secretes ▪ All individuals in a colony are connected to each other by blood vessels, through which they share food o All have complete guts o Fertilized eggs develop into larvae ▪ When a larva settles to the bottom and metamorphoses, it forms the first member of a colony; the rest are formed by asexual reproduction of that first individual o All aquatic (mostly marine) • Brachiopods: ~335 species o All unitary and coelomates o All use a lophophore for suspension feeding and gas exchange o Look like clams; have two shells made of calcium carbonate or calcium phosphate ▪ Lophophore and the rest of the body is inside these shells o Most have incomplete guts o All marine. Not very diverse; were common in Paleozoic (~250 mya) but not so much now • Molluscs: ~100,000 species o All unitary and coelom
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