98 



EECEEATIVE SCIENCE. 



process of separation, for it even proceeded 

 through, the centre of the snout to the ter- 

 minal pointed process, at the root of which 

 it turned off on the right side, so that the 

 least effort of the animal was sufficient to 

 afford it a passage." 



Preparatory to this exuviation or moult, 

 it would appear that the flesh within the 

 great claws is astonishingly reduced in vol- 

 ume and becomes very soft, parting from the 

 thin, semi-osseous internal plate, which comes 

 away attached to the joint of the formidable 

 pincers so that it slips out with little difficulty. 



In the river crayfish the mode of exuvia- 

 tion is different, and not only tedious, but, 

 according to E-eaumur, even painful. 



In the common edible crab. Cancer pagu- 

 mis, the exuviation, according to Mr. Couch 

 (an admirable observer), takes place by a 

 separation of the backplate, or carapace, 

 from the under investment, the animal lying 

 on its back during the process. 



Previously to this process, as in the lob- 

 ster and others, the fleshy contents of the 

 limb-cases shrink very considerably ; other- 

 wise the flesh of the great pincers {cJielce), in 

 particular, could not be extricated, for it does 

 not appear that the shells of these chelcB, 

 either in the crab or lobster, are fissured. 



"The newly extricated crab, not unlike 

 a lump of dough inclosed in membrane, has, 

 at first, only strength enough to enable it to 

 crawl to a place of safety — some crevice or 

 hole. There it absorbs as much water 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 deposition of the new calcareous crust is 

 made according to the acquired bulk of the 

 animal, which is proportionally the most in- 

 creased in the youngest individuals." In 

 the earlier stages of life, the exuviation and 

 sudden pushing forward of growth occur 

 several times in the course of the year, but 

 at more distant intervals as the animal verges 

 ' towards maturity. 



The drawing represents the carapace, 

 or large backplate, of a crab, covered 

 with oysters, one or two of which are at 

 least from four to five years old, and have 

 there remained since the time in which they 

 were deposited as mere dots on the spot 

 where they have grown undisturbed, until 

 dredged up by the fisherman. This crab 

 was alive when we obtained it, and so were 

 the oysters, and it was a curious sight to see 

 the crab crawling about with its living but 

 oppressive burden, the weight of which, on 

 terra firma, or, rather, the board of the 

 fishmonger, evidently incommoded it. On 

 this shell five large oysters were firmly fixed, 

 so as almost to cover the whole surface of 

 the carapace. Yet the crab was by no 

 means large — certainly not fully adult, its 

 breadth across being only seven inches. 

 Besides the oysters, there was a sandy de- 

 posit of considerable extent and elevation 

 obscuring the head and the intervals be- 

 tween mollusc shells, and passing underneath 

 their elevated edges. This sandy deposit, 

 the particles of which were firmly agglu- 

 tinated together, and which, on a prima facie 

 view, resembled a sandy ant-hill, was the 

 work of minute sea- worms, and was, in fact, 

 a maze of burrows, which these sea-worms 

 had constructed, and in which they dwelt, as 

 in a populous city, and from which, when in 

 the water, they half-extended themselves, 

 twirling and twisting in quest of their ani- 

 malcule food. The sight must have been 

 very beautiful; but, when we obtained the 

 specimen, though the crab and the oysters 

 were alive, the delicate gelatinous sea-worms 

 had all perished. Still their masonry re- 

 mained to testify as to their long residence 

 on a congenial basis. 



On considering this crab attentively, the 

 following particulars struck us. It was not 

 full-grown, it was not developed, and yet 

 for four or five years it had not cast its cal- 

 careous armour. This armour, or at least 

 the backplate, was thinner than usual, and 



