FISHES 271 



first is supported by exceedingly sharp spines, of an externally 

 symmetrical (homocercal) caudal fin shaped like a fan, and an 

 anal fin, quite close to which is the external opening of the 

 intestine, for there is here no cloaca. Fan-shaped pectoral and 

 pelvic fins are present, of the rayed type found in living Ganoids 

 except the Bichir and Reed- Fish, and it may be noted that the 

 latter are placed very far forward, much more so than in many 

 other Teleosts. The lateral line, marking the position of im- 

 portant sense-organs (see p. 263), is indicated by a dark streak 

 running along either side of the body about the level of the 

 eye. The body is covered by thin overlapping scales. 



The internal skeleton is complicated by the presence of a 

 large number of bones, and contrasts strongly with the com- 

 paratively simple cartilaginous skeleton in Dog- Fish (see p. 259). 

 The skull is particularly elaborate, and it must suffice to mention 

 once more the presence of tooth-bearing bones bordering the 

 jaws, which are attached at the back to the main skull by a 

 hyo-mandibular bone equivalent to the cartilage of the same 

 name in Dog-Fish (see p. 260). And as the Dog-Fish possesses 

 no operculum, it has nothing corresponding to the bones which 

 support that structure here. 



The skeleton by which the base of the tail is supported 

 presents some points of great interest. It will be remembered 

 that in the Dog-Fish the tail is unsymmetrical (heterocercal), the 

 vertebral column bending upwards and running into its upper 

 lobe. Here, on the contrary, the expanded tail is externally 

 symmetrical (homocercal), and the backbone apparently stops 

 short at its base. Examination of certain very young Teleosts, 

 however, shows that in them the tail is unsymmetrical, the back- 

 bone bending up in the characteristic way, and close examination 

 of the apparently symmetrical tail of the adult shows that behind 

 the last joint of the backbone is a little bony rod which turns 

 upwards, and marks what is really the posterior end of the 

 body. The homocercal tail, therefore, is really a modification 

 of the old-fashioned heterocercal one, and may be looked upon 

 as an improved type which has arisen from it. 



Digestive Organs (fig. 163). — Small pointed teeth are present 

 on the roof of the mouth-cavity as well as on the margins of 

 the jaws, and the usual regions of the gut are present, i.e. 

 mouth -cavity itself, pharynx out of which the gill -slits open, 



