CHAPTER V 



GADUS, A CHORD ATE WITH BONE 



Externals. — The genus Gadus includes the cod, whiting, and 

 haddock. In shape, Gadus differs from the dogfish in being 

 relatively shorter and more compressed from side to side. 

 Gadus belongs to the group of higher bony fish known as the 

 Teleostei, and in these the tail is typically forked and outwardly 

 symmetrical, a condition called homocercal. In Gadus the 

 tail is also outwardly symmetrical, but the tail-fin differs from 

 that of other Teleostei. It represents the hind ends of the 

 dorsal and ventral median fins. It is therefore not a homo- 

 cercal but a pseudocaudal fin (see p. 325). 



There are three dorsal and two ventral median fins. Of the 

 paired fins the pelvic pair is actually anterior to the pectoral 

 pair in position. 



The mouth is bounded by tooth-bearing jaws. On the 

 upper side of the snout, slightly behind the mouth, are the 

 nasal pits. Each of these is a cavity communicating to 

 the exterior by two openings, but not in any way connected 

 with the mouth. 



The eyes are large. The gill-slits do not open separately 

 to the exterior, but they are covered over by an operculum. 

 The water which emerges from the gill-slits passes between 

 the hind and lower edges of the operculum and the body. 

 There is no open spiracle. The cloaca is shallowed out, so 

 that the anus and the urino-genital apertures are separate ; 

 the former in front of the latter. 



Scales.' — Scales form one of the most obvious features of 

 the fish ; they are arranged in W-shaped rows, overlapping 

 from head to tail. Each row primitively corresponds to the 



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