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 every day 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. 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 that 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, exuviation is effected in the course of a few hours, body and limbs being alike relieved from their hard covering. Until the new shell acquires firmness and strength, the creature is very shy, and in the state of nature, retires into cavities below rocks or heaps of protecting sea-weed. Sir John had kept for some time one of our smaller species of shore-crabs (_Carcinus monas_), of medium size, of a brown color, with one white limb. One summer evening it was put outside the window in a capacious glass-vessel of sea-water. In the morning a form exactly resembling its own, only somewhat larger, lay in the vessel. This was the same animal, which had performed exuviation, and extricated itself from the old shell during the night. The resemblance between both forms was complete--every thing was the same, even the white limb was seen in both. Another specimen kept was of smaller size, the opposite extremities of the limbs being only thirteen lines asunder; its color was green, with three white patches on the back. In the course of little more than a year five exuviations took place at irregular intervals, the new shell and animal becoming larger each time. The third shell came on uniformly green, the white spots being entirely obliterated. On the fourth exuviation, the limbs expanded two inches