15. Earland blastopore archenteron (primitive gut) Future anus mouth Blastopore becomes the mouth and the anus forms secondarily
16. Earland blastopore archenteron (primitive gut) Future mouth anus Blastopore becomes the anus and the mouth forms secondarily
17. Earland 2 cells 4 cells 8 cells Blastomeres divide at an oblique angle to one another, so that each lies in the furrow created by the cells beneath them
18. Earland 2 cells 4 cells 8 cells Blastomeres divide in a symmetrical fashion, producing layers of cells directly on top of one another
19. Earland 4-cell stage One blastomere is removed Development is arrested (or is defective)
20. Earland 4-cell stage One blastomere is removed Development continues each blastomere is capable of regulating its development even when separated from the others Development continues
21. Earland Porifera No true tissues True tissues 2 tissue layers; radial symmetry Ctenophora Cnidaria 3 tissue layers; bilateral symmetry Platyhelminthes No body cavity Body cavity Pseudocoel Nematoda Rotifera Coelom Protostome development Annelida Mollusca Insecta Deuterostome development Mammalia Echino- dermata
Biology: Life on Earth (Audesirk) Chapter 22 (a) Cnidarians and flatworms have no cavity between the body wall and digestive tract.
Biology: Life on Earth (Audesirk) Chapter 22 (b) Roundworms are pseudo-coelomates.
Biology: Life on Earth (Audesirk) Chapter 22 (c) Annelids have a true coelom.
Biology: Life on Earth (Audesirk) Chapter 22 An evolutionary tree of some major animal phyla
Biology: Life on Earth (Audesirk) Chapter 22 Flatworms such as planarians have well-developed organ systems. (a) The elaborately branched digestive system, the centrally located ventral pharynx, and eyespots in the head are clearly visible. (b) (Left) The excretory system consists of branching tubes that conduct excess fluid to the outside through numerous pores. Cilia keep the fluid moving. (Right) The nervous system of flatworms shows clear cephalization, with eyes and a brain composed of ganglia cells in a well-defined head. Ladderlike nerve cords carry signals through the rest of the body.
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