Animalia I--Sponges Through Echinoderms
24.1 What Is an Animal?
1. Animals are multicellular eukaryotes whose cells do not have cell walls. Vertebrates have backbones; the more abundant invertebrates do not.
2. Animals have immature stages, and are motile at some point in the life cycle.
3. An immediate animal ancestor was likely a choanoflagellate.
4. The earliest fossil evidence of animals is from 580 to 570 million years ago.
24.2 What Characteristics Define and Distinguish Animals?
5. Animal cells secrete an extracellular matrix.
6. Animals are heterotrophs and may be carnivores, herbivores, or detritivores.
7. Animal bodies exhibit degrees of organization into tissues, organs, and organ systems.
8. Body symmetry may be radial with an oral end and aboral end, or bilateral, with a cephalized (head) end.
9. An animal zygote undergoes cleavage divisions to form a blastula and then a gastrula, which has ectoderm, endoderm, and in some species, mesoderm. Those with two layers are diploblastic; with three layers, triploblastic.
10. Animals may undergo direct development or metamorphose from a larva to an adult. They reproduce sexually and sometimes asexually.
11. A coelom is a body cavity lined with mesoderm. Animals may be coelomate, acoelomate, or pseudocoelomate.
12. Major coelomate lineages differ in developmental pattern. Protostomes have spiral and determinate cleavage, and a blastopore developing into a mouth. Deuterostomes have radial and indeterminate cleavage and a blastopore developing into an anus.
24.3 The Sponges (Porifera) Are the Simplest Animals
13. Sponges are asymmetrical or radially symmetrical suspension feeders that move slowly.
14. Pinacocytes, amoebocytes, archeocytes, and choanocytes are sponge cells. They do not form true tissues.
15. A sponge’s skeleton consists of spicules within spongin.
16. Sponges reproduce sexually and may bud asexually using gemmules.
24.4 Cnidaria Are Radially Symmetrical, Aquatic Invertebrates
17. Cnidaria include jellyfishes, corals, and Portuguese men-of-war.
18. These animals capture prey with stinging nematocysts released from cnidocytes.
19. Cnidaria have radial symmetry, are diploblastic, and partially digest food in a gastrovascular cavity.
20. A cnidarian body form is a polyp or a medusa. Fertilization in water produces a planula, which attaches and becomes a polyp that reproduces asexually to generate a strobila, which yields medusae.
24.5 Flatworms (Platyhelminthes)
21. Flatworms include planaria, flukes, and tapeworms.
22. Flatworms are triploblastic, lack coeloms, and have bilateral symmetry. Protonephridia maintain internal water balance.
23. Most flatworms are parasitic and have great reproductive capacity.
24. Flatworms reproduce asexually and sexually, and some are hermaphrodites.
24.6 Roundworms (Nematoda)
25. Roundworms include parasitic and free-living species in soil or aquatic sediments.
26. Nematodes are bilaterally symmetrical, cylindrical, and unsegmented.
27. Nematodes have pseudocoeloms, complete digestive systems, and separate sexes.
24.7 Mollusks Include Clams, Snails, Octopuses, and Squid
28. The diverse mollusks have mantles, muscular feet, visceral masses, and complex organ systems.
29. Mollusks are protostomes with bilateral symmetry and have trochophore larvae.
30. Sexes are separate in the mollusks.
24.8 Annelids Are Segmented Worms
31. Annelids include oligochaetes, characterized by setae (bristles) and a clitellum; polychaetes, with parapodia as appendages; and the hirudinea (leeches) with annuli between their segments.
32. Annelids feed in diverse ways and have organ systems.
33. Leeches and oligochaetes are hermaphroditic; polychaetes have separate sexes.
24.9 Arthropods Include Insects, Crustaceans, and Arachnids
34. Arthropods are protostomes with bilateral symmetry, and they are segmented with jointed appendages and a chitinous exoskeleton.
35. Arthropods have open circulatory systems, spiracles to respire, and a nervous system.
36. Four subphyla include trilobites; horseshoe crabs and arachnids; crustaceans; and insects and myriapods.
24.10 The Echinoderms: Life Based on Five-Part Symmetry
37. Echinoderms are spiny-skinned marine animals with pentaradial symmetry that move using water vascular systems to power tube feet.
38. Similarities in embryonic development and molecular evidence place echinoderms closest to chordates.
39. Echinoderm larvae look very different from adults and display bilateral symmetry.