. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 236 CLASSIFICATION OF Fig. 238. Section through the broadly expanded medullary plate of a shark {^Acanthias'). a, archenteron; c, coelom; vi, meso- thelium ; w/, medullary plate; «, notochord. the edges of this rise up and gradually unite, so that the plate becomes converted into the medullary tube. Gradually the embryo thus out- lined is raised above the yolk, and soon becomes so separated from it that only a slender yolk stalk re- mains connecting the two. This stalk carries blood-vessels (ompha- lomesaraics)
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 236 CLASSIFICATION OF Fig. 238. Section through the broadly expanded medullary plate of a shark {^Acanthias'). a, archenteron; c, coelom; vi, meso- thelium ; w/, medullary plate; «, notochord. the edges of this rise up and gradually unite, so that the plate becomes converted into the medullary tube. Gradually the embryo thus out- lined is raised above the yolk, and soon becomes so separated from it that only a slender yolk stalk re- mains connecting the two. This stalk carries blood-vessels (ompha- lomesaraics), while the yolk itself is connected with the alimentary canal. The mesothelium arises as in- growths on either side at the point of differentiation of ectoderm and entoderm, these growing in be- tween the two layers. The gill slits break through the sides of the neck in regular succession from in front backwards, the mouth breaking through after all five gill slits are open. For a time the gill filaments protrude from the gill slits. The spi- racle is at first the largest of the clefts, but it soon begins to close at the \-entral end so that only the dorsal portion persists. The paired fins start as lateral folds (in some cases continuous, , Acanthias), into which grow cells from the myotomes (p. 110). Elasmobranchs are, on the whole, the most primitive of the jawed vertebrates, although in some respects they seem to stand above the other fish-like forms. The sharks are free-swimming forms, seek- ing their prey in all parts of the sea, a few species ascending rivers, and one being found in Lake Nicaragua. The skates, on the other hand, are bottom feeders, living on molluscs, crabs, etc., and their teeth are modified into crush- ing-plates adapted to such food. None of the species are very small; but some of them are of enormous size, among the largest. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readabilit
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