236 



CLASSIFICATION OF VERTEBRATES. 



FIG. 238. Section through the 

 broadly expanded medullary plate 

 of a shark {Acantkias}. a, 

 archenteron ; c, ccelom ; m, meso- 

 thelium ; w/, medullary plate ; n, 

 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 ventral end so that only the dorsal portion persists. 

 The paired fins start as lateral folds (in 

 some cases continuous, e.g., Acanthias), 

 into which grow cells from the myotomes 

 (p. no). 



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 



FIG. 239. Head of 

 embryo Acanthias, show- 

 ing the gill filaments pro- 

 truding from the gill slits. 



