FORM AND EXTERNAL FEATURES. 495 



communication between tliis dorsal nervous tube above and 

 the ventral alimentary tube persists for some time as the 

 neurenteric canal (Fig. 164, n.e.c). 



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 estabhshed (Fig. 164, «.). 



Besides the internal establishment and differentiation of 

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

 the blastoderm around the yolk, (/') 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 absorption. 



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

 may be inferred from the general conclusions stated else- 

 where. 



Second type of Fishes. The Haddock {Gadus ceglefinus) 

 — A type of Teleosteans with closed swim bladder 

 (Physoclisti). 



Form and External Features. 



The elongated wedge-like form is well adapted for rapid 

 swimming. The terminal mouth bears a short barbule ; 

 this is long in the cod {G. morrhtia), and absent in the 

 whiting {G. merlangus). The nostrils, situated near the 

 end of the snout, have double apertures. The eyes are lid- 

 less, but covered with transparent skin. Over the gill 

 chamber and the four gills lies the operculum, supported by 

 several bones. Distinct from one another, but closely 

 adjacent, are the anal, genital, and urinary apertures, — 

 named in order from before backwards. Along the sides of 

 the body runs the dark lateral line containing sensory cells. 

 There are three dorsal and two anal fins, and an apparently 

 symmetrical tail fin. 



