ORGANS OF OFFENCE AND DEFENCE. 133 



portant limitation. The chain of developement 

 is not unbroken through the orders of animals, from 

 man down to the mollusca, Crustacea, and insects ; 

 the developement takes place at two extremes, the 

 middle point between which is occupied by the 

 order of fishes, in which this expansion is entirely 

 deficient. In birds, the same muscular expansion 

 attains but a trivial importance ; and, of the rep- 

 tiles, the Oj)hidia, or serpents, alone show faint 

 traces of it. 



Some popular writers on comparative anatomy 

 have made a statement liable to mislead, connected 

 with the same organ, in representing the globular 

 form assumed by the Diodons and Triodons, and 

 the erection of the numerous spines with which the 

 surface of their bodies is beset, which happens when 

 they are in danger, as analogous to the erection of 

 the spines of the hedge -hog when it gathers itself 

 into a ball. The analogy so far holds, that in the 

 case of both the spines become erected as organs of 

 defence when any danger appears ; but in the fishes, 

 the distension of the skin is produced by a general 

 enlargement of the whole body, consequent on the 

 reception of air into the crop or first stomach ; while, 

 in the hedgehog, the erection of the spines is pro- 

 duced by the action of the musculai* organ before 

 refeiTcd to, an appendage of the panniculus car- 

 nosus. 



To return to the skin itself, in this order of ani- 

 mals, it has little of a fibrous character, approaching 

 more to the mucous texture ; its tissue is by no 



