426 FISHES. 



Along the mid-dorsal line of the epiblast a medullary 

 groove appears — the beginning of the central nervous 

 system. Its sides afterwards arch towards one another, and 

 meet to form a medullary canal. A posterior communica- 

 tion between this dorsal nervous tube above and the ventral 

 alimentary tube persists for some time as the neurenteric 

 canal. 



The mesoblast arises as two lateral plates, one on each 

 side of the medullary groove. The plates seem to arise as 

 a pair of solid outgrowths from the wall of the gut. They 

 are afterwards divided into segments. Between the meso- 

 blast plates, along the mid-dorsal line of the gut, the noto- 

 chord is established. 



Besides the internal establishment and differentiation of 

 layers, there are two important processes, (a) the growth of 

 the blastoderm around the yolk, (b) the folding off of the 

 embryo from the yolk. The yolk is thus enclosed in a yolk- 

 sac, with which the embryo is finally connected only by a 

 thin stalk — the umbilical cord. Through the canal of this 

 cord nutriment is absorbed into the gut, and blood-vessels 

 also effect the same result. 



What the different layers form, and how the organs arise, 

 may be safely inferred from the general conclusions stated 

 elsewhere. 



THE ORDERS OF FISHES. 



Order I. Elasmobr-inchii — Cartilaginous Fishes. 



Synonyms. Selachii. Plagiostomata (with transverse 

 ventral mouth). 



Sharks and skates represent the two distinct types included 

 in this order. They are voracious carnivorous fishes. The 

 scales are " skin-teeth." There is no cover over the (5-7) 

 gill-apertures; anterior to these there is often a spiracle, 

 — the first gill-cleft with a rudimentary gill. The fins 

 are large. The skeleton is mostly cartilaginous. The tail 

 is asymmetrical or heterocercal. The mouth extends trans- 

 versely on the under side of the head. The nostrils are 

 also ventral. A spiral fold extends along the internal wall 

 of the large intestine. Into the terminal chamber (or 



