Elementary textbook of economic zoology and entomology . elementarytextbo00kell Year: [c1915] 238 ECONOMIC ZOOLOGY AND ENTOMOLOGY themselves to a rock or some other firm substance, and all of their organs become very much reduced and simplified. The adult ascidian is a degenerate, sac-like organism looking as much like a plant as an animal, and showing in no way the relation to the vertebrates that is suggested by the larva. As these two classes are so unlike each other and so different from the vertebrates they are often considered as two distinct subbranches of the Chordata and all the othe
Elementary textbook of economic zoology and entomology . elementarytextbo00kell Year: [c1915] 238 ECONOMIC ZOOLOGY AND ENTOMOLOGY themselves to a rock or some other firm substance, and all of their organs become very much reduced and simplified. The adult ascidian is a degenerate, sac-like organism looking as much like a plant as an animal, and showing in no way the relation to the vertebrates that is suggested by the larva. As these two classes are so unlike each other and so different from the vertebrates they are often considered as two distinct subbranches of the Chordata and all the other classes are included in a third subbranch, the Vertebrata. In almost all FIG. in.—An ascidian or sea-squirt from the coast of California. (After Jordan and Kellogg.) of the Vertebrata, the notochord, which is present in the early stages of development, is replaced, in the later stages, by 'a cartilaginous or bony backbone or spinal column. The class Leptocardii (Gr. Icptos, small; kardia, heart) includes the primitive lancelet, in which the notochord is persistent and unsegmented. Lancelets occur in the sand in shallow water
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