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 (Siluridae), 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 



B 



FIG. 804. Stomias boa. The white dots are the luminous organs. (From Hickson, 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 



g 



