B. SKELETON. 



1. EXOSKELETON. 



THE hard exoskeleton, consisting of bone or of other calcified 

 tissues, must be distinguished from the horny exoskeletal parts 

 already described. Thus it will be remembered that the term 

 " scale " is sometimes used for a horny epidermal structure, and 

 sometimes for a bony dermal one (pp. 20 and 23). 



In Cyclostomes, scales are entirely wanting. 



Elasmobranchs. The integument of most Elasmobranch 

 Fishes encloses certain hard structures each consisting of a basal 



FIG. 30. PLACOID SCALES FROM THE SKIN OF AN ELASMOBRANCH. 



(Semi-diagrammatic. ) 



S, basal or socket-plates in the dermal connective tissue (By) ; X, 



denticles. 



plate or socket bearing a pointed spine or denticle (Fig. 30), differ- 

 ing considerably in form and relative size in the various members 

 of the Order, and known as a placoid scale. These placoid organs 

 are continually being formed anew throughout life, and are pro- 

 tective in function. The basal plate is rhomboid or rounded in 

 form and consists of bony tissue, while the denticle itself is 

 composed of dentine covered over with enamel. In many Rays, 

 there is a relatively small number of these placoids, while in most 

 Sharks and Dogfishes they are much more numerous and close- 

 set. In the Electric Rays they are wanting. The primary part 



