60 THE VERTEBRATE SKELETON. 



usually more than one membrane bone developed in connection 

 with each half. 



There are at least four pairs of branchial arches present 

 during development. The sternum, if present, is not costal 

 in origin. 



CLASS I. PISCES. 



The exoskeleton is in the form of scales, which may be 

 entirely mesoblastic or dermal in origin (e.g. cycloid and 

 ctenoid scales), or may be formed of both mesoblast and epi- 

 blast (e.g. placoid and ganoid scales). Large bony plates may 

 be derived from both these types of scale. In general fish 

 with a greatly developed dermal armour have the endoskeleton 

 poorly developed; and the converse also holds good. 



The integument of the dorsal and ventral surfaces is 

 commonly prolonged into longitudinal unpaired fins, sup- 

 ported by an internal skeleton. These fins are distinguished 

 according to their position as dorsal, caudal and anal fins. 

 The dorsal and anal fins are used chiefly as directing organs, 

 the caudal fin is however a most important organ of pro- 

 pulsion. 



Three types of tail are found in fishes, viz. : 



1. The diphycercal, in which the axis is straight and 

 the tail is one-bladed and symmetrical, an equal proportion of 

 radialia 1 being attached to the upper and lower surfaces of the 

 axis. 



2. The heterocercal, in which the tail is asymmetrical 

 and the axis is bent upwards, the proportion of radialia or of 

 fin-rays attached to its upper surface being much smaller than 

 that attached to its lower surface. 



3. The home-cereal, in which the tail though ex- 

 ternally symmetrical, so far resembling the diphycercal type, is 

 internally really heterocercal, the great majority of the radialia 

 or of the fin-rays being attached to the lower surface of the axis. 



1 See p. 79. 



