CLASS I 



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 brauch 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 rule on flat cartila- 



giuous 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, Eaja) every 

 gradation can be observed between the Placoid scales 

 of the externa! 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 granulär 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 Foriheus. Among the UpperT 



sharks there often occur teeth shaped 



like an arrow-head, flattened on one side, with or without 



lateral denticles. To crush the food there are sometimes 



obtusely conical teeth, sometimes bean-shaped, hemispherical, 



or pavement-like teeth, and in certain Selachii the teeth are 



closely pressed together into a continuous mosaic. Peculiar 

 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. 

 AVhile the Dipnoi, Chimaeroidei, and Gymnodonts possess only four to ^ 



Fio, 10. 



Dorsal fin spine of Ilybodiis. 

 Aas ; Boll, Würteinberg. 



Fig. 11. 



.-1, B, Rays from the 

 dorsal fin of an Acan- 

 thopterygian. C, Fin 

 snpport or interspin- 

 ons bone. 



