THE EXOSKELETON. 



41 



The dermal cxosl^eleton arises from the liardening of the 

 dermis ; in the majority of cases by the deposit of bone-earth, 

 in more or less completely-formed connective tissue, though 

 the resulting hard tissue has by no means always the struct- 

 ure of bone. It may happen that cartilage is developed in the 

 dermis ; and, either in its primary state or ossified, gives rise 

 to exoskeletal parts. 



Fig. 16. — A, outline of a Pike (Ksocr), to show the fins : P, pectoral ; F, ventral ; J, anal ; 

 C caudal; Z*, dorsal, fins. Op., operculum; P.Op., preoperculum ; £i\ brancliiostegal 

 rays. — B, scales of the dermal exoskeleton of the same fish. 



No dermal exoskeleton (except that of the fin-rays) is found 

 m the lowest fishes, Am^jhioxiis and the 3Iarsipohrancliii. In 

 most Teleostei, the integument is raised up into overlapping 

 folds ; and, in these, calcification takes place in lamina3, of 

 which the oldest is the most superficial, and lies immediately 

 beneath the epidermis. As a general rule, the calcified tissue 

 of the " scale " thus formed, does not possess the structure of 

 true bone in the Teleostei. But, in other fishes, the dermal 

 calcification may consist of true bone (as in the Sturgeon) ; or, 

 as in the Sharks and Rays, may take on the structure of teeth, 

 and consist mainly of a tissue exactly comparable to dentine, 

 capped with enamel, and continuous by its base with a mass 

 of true bone, which takes the place of the crusta x>etrofia^ or 

 cement of the teeth. 



A form of dermal exoskeleton, which is peculiar to and 

 highly characteristic of fishes, is found in the Jin-rays. These 

 are developed in the integument either of the median line of 

 the body, or in that of the limbs. In the former case, they 

 usually enter into, or support, folds of the integument which 

 are termed dorsal^ caudal, or anal fins — according as they lie 



