3490 Crustacea. 



Exuviation of the Crustacea. — " The crustacean class of animals, of which the 

 lobster, crab, and shrimp are familiar examples, have this peculiarity of structure — 

 that their soft bodies are inclosed within a coat of mail formed of carbonate and phos- 

 phate of lime. In fact, they carry their skeleton outside their bodies, both for defence 

 of the vital parts within, and for the attachment of the muscles which move their limbs, 

 and every part of their frame. No warrior of old was ever more completely enveloped 

 in his hard coat of mail, with its jointed greaves and overlapping scales, than is the 

 lobster in its crustaceous covering; with this exception, that the warrior could at plea- 

 sure unbuckle himself from his armour, whereas the body and limbs of the Crustacea 

 are completely incased in hollow cylinders, firmly and accurately jointed, from which 

 there is no such ready release. Now, as this shelly integument envelopes them from 

 their earliest youth, and as it does not expand and grow, the natural growth of the soft 

 body beneath would be entirely prevented, did not nature supply a remedy of a very 

 curious kind — the exuviation, or periodical throwing off of the external crust, and the 

 formation of a larger shell-covering, fitted for the increasing growth of the animal. 

 This is a circumstance which has long been familiar to naturalists, and indeed the 

 most ordinary observer must have often remarked in the crabs and lobsters brought to 

 table, appearances indicative of their change of external coverings. In the back of 

 the edible crab, may often be noticed a red membrane lining the inner side of the 

 shell, but so loose as to be readily detached. Along the greater part of its course this 

 membrane has already assumed a half-crustaceous consistence, and is just the prepa- 

 ratory process to the old shell being thrown off by the animal. There is another curi- 

 ous circumstance which has also been long known — that crabs and lobsters can renew 

 lost limbs. Some misconceptiou, however, had existed regarding the manner in which 

 this was effected, until the observations of the late Sir John Dalyell have thrown more 

 accurate light on the subject. This most amiable and eminent zoologist, who was lost 

 to science last year, afforded a pleasing illustration of the solace and delight which the 

 pursuit of the study of Nature yields to the diligent inquirer into her mysteries. With 

 a feeble constitution and frame of body, which precluded his mingling in the more ac- 

 tive pursuits of everyday life, this sedentary philosopher collected around him exam- 

 ples of minute and curious being from the depths of the ocean, from lake and river, 

 and for many long years found the delight of his leisure hours in watching the habits 

 of the animals, and in discovering and describing many singular circumstances in the 

 constitution of their bodies, and the peculiar adaptation of their structure and instincts 

 to their modes of existence. One of his last communications to the public, imparted 

 with all the modesty and simplicity of true genius, at the last meeting of the British 

 Association in Edinburgh, was on this subject of the exuviation of the Crustacea* 

 It appears from Sir John's observations that crustaceans begin to throw off their shells 

 at a very early period of their life, even in that embryo state in which they first appear 

 after having left the egg, and before they have yet assumed the real form of their 

 mature state. During every successive exuviation in this embryo state, they assume 

 more and more of their perfect and established form. While the crab is young and 

 rapidly growing, frequent exuviations take place at short intervals, from three to five 

 times in the course of one year. Previous to the change, the animal almost ceases to 

 feed, and becomes rather inactive ; the proper time having at length arrived, exuvia- 



' Report of British Association, 1851.' Pp. 120-122. 



