THE PLACOID BRAIN. 143 



and mammals, and so unique in the animal kingdom, that 

 the heated water in which the one readily dissolves has no 

 effect whatever upon the other. It is, however, a curious cir- 

 cumstance, exemplified in some of the shark family,* though 

 it merely serves, in its exceptive character, to establish the 

 general fact, that while the rays of the double fins, which 

 answer to the phalanges, are all formed of this indissoluble 

 cartilage, those rays which constitute their outer framework, 

 with the rays which constitute the framework of all the single 

 fins, are composed of a mucoidal cartilage, which boils into 

 glue. At certain definite lines a change occurs in the tex- 

 ture of the skeleton ; and it is certainly suggestive of thought, 

 that the difference of substance which the change involves 

 distinguishes that part of the skeleton which is homologically 

 representative of the skeletons of the higher vertebrata, from 

 that part of it which is peculiar to the creature as a fish, 

 viz. the dorsal and caudal rays, and the extremities of the 

 double fins. These emphatically ichthyic portions of the 

 animal may be dissipated by boiling, whereas what Linnaeus 

 would perhaps term its reptilian portion abides the heat with- 

 out reduction. 



But is not the one-sided tail, so characteristic of the sharks, 

 and of almost all the ancient ganoids, also a characteristic of 

 the young salmon just burst from the egg ] Yes, assuredly; 

 and, so far as research on the subject has yet extended, of 

 not only the salmon, but of all the other osseous fishes in 

 their foetal state. The salmon, on its escape from the egg, is 

 a little monster of about three-quarters of an inch in length, 

 with a huge heart-shaped bag, as bulky as all the rest of its 

 body, depending from its abdomen. In this bag provident 

 nature has packed up for it, in lieu of a nurse, food for five 

 weeks ; and, moving about everywhere in Hs shallow pool, 



* Such as the dog-fishes, picked and spotted. 



