A CURIOSITY IN NATURAL HISTORY.

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 enclosed within a coat-of-mail formed of carbonate and phosphate 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 pleasure 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 envelops 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 preparatory process to the old shell being thrown off by the animal.  There is another curious circumstance which has also been long known—­that crabs and lobsters can renew lost limbs.  Some misconception, 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 active pursuits of everyday life, this sedentary philosopher collected around him examples 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 adaptations 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.[2]