PISCES 



structures. They arise as folds of skin, which gradually become strengthened 



by numerous horny fibres (Selachii, Dipnoi). In most Ganoids and Tele- 



osteans these fibres are replaced by bony rays, which 



consist of two closely apposed halves. In the Acan- 



thopterygii (Fig. 11) each half consists of a single 



piece ; in the Malacopterygii the rays are subdivided 



by transverse sutures into numerous little pieces and 



often branch towards the end. Spiny and articulated 



rays frequently occur in one and the same fin, and in 



this case the spines are always in front. The rays of 



the unpaired fins are borne as a I'ule on flat cartila- 

 ginous or bony supports with which they articulate 



(Fig. 11, C). These fin supports penetrate between 



the spinous processes of the vertebrae and are known 



as axonosts or (according to their position) as inter- 



neurals and interhaemals. 



In many Selachians (Pristiophorus, Raja) every 



gradation can be observed between the Placoid scales 



of the external skin and the teeth ; and as both 



exhibit essentially the same histological structure, 



while the teeth only assume a firm connection with 



the head bones in the higher fishes, the teeth must 



indeed be regarded as dermal structures. 



Among fishes all the cartilages or bones surround- 

 ing the mouth and gill cavity may 

 bear teeth. They originate in the 

 calcification of skin papillae, con- 

 sisting of a crown which freely 

 projects, and of a root which is 

 surrounded with connective tissue 

 or united with the head bones. 

 Their form and proportions are 

 extraordinarily variable according to 

 their functions. All gradations exist 

 from the minute granular denticles of 

 the perch to the long brush-like teeth 

 of the sheat-fish, to the robust pre- 

 hensile teeth of the pike and the 

 powerful conical or laniary teeth of 



Dendrodus or Poriheus. Among the Upper i.ias ; iBoii, Wurtemijerg, 

 sharks there often occur teeth shaped 

 Fig. 11. ^^^6 an arrow-head, flattened on one side, with or without 



A, B, Rays from the lateral dcnticles. To crush the food there are sometimes 



thoptery'iian.^'^c^^Fiii obtusely conical teeth, sometimes bean-shaped, hemispherical, 



support or interspiii- or pavcmcnt-like teeth, and in certain Selachii the teeth are 



ous bone. '- . . ■ -r^ t 



closely pressed together into a continuous mosaic, reculiar 

 cutting dental plates of considerable size occur in Diodon, Arthrodira, and 

 Chimaera. 



The number of the teeth in fishes is also as varied as their shape. 

 While the Dipnoi, Chimaeroidei, and Gymnodonts possess only four to 



Fig. 10. 

 Dorsal fin spine of Uybodus. 



