344 Ichthyology of the Firth of Forth. 



off to a place of safety, commonly beneath the shelter of a neighbour- 

 ing crevice, of such limited dimensions at its entrance, that it seems 

 surprizing it should ever be able to return from it, in its new state 

 of growth. The first instinct of nature after securing a place of 

 retreat, is to absorb, I suppose by swallowing it, as much fluid as 

 will distend its organs and their common covering, now as flexible 

 as velvet, to the full extent of their capacity ; by which means the de- 

 position of crustaceous particles is made according to the dimensions 

 of its newly acquired bulk : which in the instance now described, 

 that had arrived at the ordinary size of a female, and allowing for 

 the thickness of the first crust, exceeded the former by at least an 

 inch in its longest diameter, and in all its dimensions in the same 

 proportion. In the early stages of growth, the increase of bulk at 

 exuviation is, of course, much more considerable, in relative pro- 

 portion. Some of these smaller specimens may be found passing 

 through this natural process in every month of the year ; but in 

 some adult individuals, the same crust must continue for a long time, 

 since I have seen one with oysters measuring two inches and a half 

 in length, on the carapace; and in another, a muscle (mytilus) about 

 an inch in length, attached by its byssus to a joint near the body. 



Whether, as has been said, the rejected case forms a meal for its 

 old inhabitant, on its first return to habits of activity, may well be 

 questioned ; though I have known an instance where one crab has 

 devoured another, leaving nothing but the points of the legs, and a 

 small portion of the carapace ; — but in the analogous instance of a 

 prawn, that had just thrown off its exuviae, I have detected the 

 source of the firmness of the new covering, in the presence of small 

 shell-fish in its stomach. Under a lens, a minute trochus, perhaps 

 the T. zizyphinus, and some specimens of an oval bivalve, were dis- 

 tinguishable. This is perhaps the only period of their lives when 

 their food is of this kind, but its nature cannot often be ascertain- 

 ed, by the comminution it undergoes from the manner in which it is 

 devoured, when the jaws have attained their firmness. 



Polperro, 1836. 



IV. — Contributions to the Ichthyology of the Firth of Forth. By 

 Richard Parnell, M. D. 



No. II.— The Tadpole Fish. 

 The object of this paper is to shew, that the Raniceps trifurca- 

 tus is by no means so rare a fish as naturalists in this country have 

 supposed, and to point out some very important characters in it, 

 which have escaped their attention. 



