XIII 



PHYLUM CHORDATA 



221 



such as the Weaver (Trachinus), possess poison-glands, opening 

 either on one of the dorsal spines, or on a spinous process of the 

 operculum, or, as in the Cat-fishes (Siluridse), on the spine of the 

 pectoral fin. 



Exoskeleton. In many Teleostomi, such as Polyodon and 

 many Eels, the skin is devoid of hard parts, but in most cases a 



\ 



FIG. 894. Stomias boa. 



The white dots are the luminous organs. (From Hicksou, after 

 Filhol.) 



dermal exoskeleton 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 overlapping 

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

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

 and Anacanthini, they are called cycloid scales (Fig. 867) ; when, as 

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

 (Fig. 895, 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. 884) there is a strong 

 armour, formed of stout bony 

 plates, or scutes, produced into 

 enamelled spines and articula- 

 ting with one another by suture. Scutes are also found in many 

 Siluroids (Fig. 887) and in Lopho branchii (Fig. 892) and some 

 Plectognathi (Fig. 891) ; 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 Polypterus and Lepidosteus 

 are found rhomboid or ganoid scales^ (Fig. 895, B), in the form 



FIG. 895. A, ctenoid scale ; B, ganoid scales. 

 (After Giinther.) 



