chap, ix.] EXTERNAL SKELETON. 363 



the fins is seen in Periophthalmus, which, thanks 

 especially to its large pectorals, is able to hop over the 

 mud. In some Gobies the ventral fins unite to form 

 a kind of suctorial disc, by means of which the fish 

 can attach itself to rocks. The sucking disc of 

 Cyclopterus lumpus is supported by the rudimentary 

 spines and rays of the ventral fins. In the flying- 

 fish (Exoccetus) the pectoral fins may extend as far 

 back as the caudal, and can be spread out so as to 

 act like sails. In cartilaginous fishes, where the 

 edges of the fins are softer than in the bony fishes, 

 these edges may perform an undulatory or screw- 

 like movement. When the lateral fins disappear, 

 the locomotor function falls altogether on the vertebral 

 column and unpaired fins. 



The External skeleton of Vertebrates is, in 

 the simpler conditions, formed by scales, which are 

 developed in the cells of the integument. The most 

 generalised condition obtains among Elasmobranch's, 

 where, as we have already learnt, the internal skele- 

 ton is throughout life cartilaginous ; in such a form 

 as the dog-fish the whole of the external surface is 

 roughened, owing to the presence of projecting 

 pointed processes, which have not inappropriately 

 been called dermal denticles, so close and strik- 

 ing is their resemblance to the processes which, when 

 placed within the area of the mouth, are called teeth ; 

 like them, they consist essentially of dentine invested 

 in a layer of harder enamel. In the huge basking 

 shark the whole of the body is covered by denticles, 

 which, taken separately, are small enough, but w r hich 

 en masse must be a very effective means of defence. 

 In the spinous shark (Echinorhinchus) the diffused ar- 

 rangement yields to one in which large spinous tuber- 

 cles are scattered over the body, and the value of that 

 diffused arrangement is very eloquently spoken to by 

 the naked body of the torpedo, which has found a still 



