228 



ZOOLOGY 



SECT. 



(lerinal exoskelcton is present. In Amia and in the majority of 

 Teleostei this takes the form, as in the Trout, of scales, ' rounded 

 plates of bone imbedded in pouches of the derm and loverlapping 

 one another from behind forwards. When the free border of the 



Fig. 883. — Stomias boa. The white dots are the himiuous organs. (Prom Hickson, after 



Filhol.) 



scales presents an even curve, as in Amia and most Physostomi 

 and Anacanthini, they are called cycloid scales (Fig. 856) ; when, as 

 in most Acanthopteri, the free edge is produced into small spines 

 (Fig. 884, A), they are distinguished as ctenoid scales. Usually 

 the integument is continued as a thin layer over the surface of 

 the scales, but in a good many cases this investment is absent. In 

 exceptional cases the scales may be so large and strong as to form 

 a rigid armour. In the Sturgeon (Fig. 873) there is a strong 

 armour, formed of stout bony plates, or scutes, produced into 

 enamelled spines and articulating with one another by suture. 

 Scutes are also found in many Siluroids (Fig. 876) and in 

 Lophobranchii (Fig. 881) and some Plectognathi (Fig. 880); while in 



other Plectognathi the exo- 

 skeleton takes the form, as 

 in the File-fishes, of minute 

 spines like the shagreen of 

 Sharks, or, as in many Globe- 

 fishes, of long, outstanding, 

 bony spines. Lastly, in Poly- 

 pterus and Lepidosteus are 

 found rhomboid or ganoid 

 scales (Fig. 884, B), in the 

 form of thick, close-set, rhom- 

 boidal plates formed of bone, 

 covered externally by a layer 

 of enamol-likc material (ganoioi) and joined together by pegs and 

 sockets. In )nany Ganoids the anterior fin-rays of both median 

 and paired fins bear a row of spine-like scales called fulcra 

 (Fig. 874,/). 





'^^^'Ife 





Pig. 8S4. — A, ctenoid scale ; B, ganoid scales. 

 (After G anther.) 



