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Animalia I--Sponges Through Echinoderms

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24.1 Introducing the Animals

1. Animals are multicellular eukaryotes whose cells do not have cell walls. Vertebrates have backbones; the more abundant invertebrates do not. Essential Study Partner
Characteristics of Animals
2. Animals have diploid cells, 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 about 570 million years ago.  

24.2 Distinguishing Features of 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. Animation
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. Two major lineages of bilaterally symmetric animals are protostomes and deuterostomes. Protostomes (Ecdysozoans and Lophotrochozoans) 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.  
12. A coelom is a body cavity lined with mesoderm. Animals may be coelomate, acoelomate, or pseudocoelomate. Art Quiz
Body Plans

24.3 The Sponges (Porifera) Are the Simplest Animals

13. Sponges are asymmetrical or radially symmetrical suspension feeders that move slowly. Essential Study Partner
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 Cnidarians Are Radially Symmetrical, Aquatic Invertebrates

17. Cnidaria include jellyfishes, corals, and Portuguese men-of-war. Essential Study Partner
Radial Animals
18. These animals capture prey with stinging nematocysts that discharge from cnidocytes.  
19. Cnidarians have radial symmetry, are diploblastic, and partially digest food in a gastrovascular cavity. Art Quiz
Cnidarian Body Plan
20. A cnidarian body form is a polyp or a medusa. In species that alternate between these forms, 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. Essential Study Partner
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 Mollusks Include Clams, Snails, Octopuses, and Squids

25. The diverse mollusks have mantles, muscular feet, visceral masses, and complex organ systems. Essential Study Partner
26. Mollusks are protostomes with bilateral symmetry and have trochophore larvae. Art Quiz
Mollusk Body Plan
27. Sexes are separate in the mollusks.  

24.7 Annelids Are Segmented Worms

28. Annelids include oligochaetes, characterized by setae (bristles) and a clitellum; polychaetes, with parapodia as appendages; and the Hirudinea (leeches) with superficial annuli within their segments. Essential Study Partner
29. Annelids feed in diverse ways and have organ systems.  
30. Leeches and oligochaetes are hermaphroditic; polychaetes have separate sexes.  

24.8 Roundworms (Nematoda)

31. Roundworms include parasitic and free-living species in soil and aquatic sediments.  
32. Nematodes are bilaterally symmetrical, cylindrical, and unsegmented.  
33. Nematodes have pseudocoeloms, complete digestive systems, and separate sexes.                      Essential Study Partner

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. Essential Study Partner
35. Arthropods have open circulatory systems, spiracles to respire, complex nervous systems, and malpighian tubules for excretion.  
36. Four subphyla include the extinct trilobites; chelicerates (horseshoe crabs and arachnids); and two subphyla of mandibulates (crustacea and urinamia). Subphylum uniramia contains the 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. Essential Study Partner
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.  

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