7052 Crustacea. 



taken having on their backs a quantity of large and well-developed 

 oysters, so large that the crab seemed almost hid by the foreign 

 intrusion. It must therefore be true that either the crab does not, in 

 its adult form, shed its skin once a year, or else that oysters grow 

 much more rapidly than they are believed to do. 



But whether it be during the larva state or that of the adult crab 

 the process of development by which the shell is produced must 

 be one and the same. Immediately above the heart a mass of 

 nucleated cells is formed, extending to the internal surface of the 

 shell, from which it is separated by a layer of pigment, which gives 

 colour to the new formation. Towards the base of this mass — that is, 

 immediately above the heart — the cells are uniformly large and 

 distinct, while an areolar tissue ramifies throughout the whole. As 

 advance is made cells of less size appear ; these increase in number, as 

 they diminish in diameter, until they approach the layer of pigment, 

 immediately beneath which they adapt themselves, by mutual com- 

 pressions, into a polygonal form. This mass of cell-structure extends 

 throughout the entire surface of the crab immediately beneath the old 

 skin, the greater or less thickness of the new mass corresponding to 

 that of the old : these cells, being the organs which collect the lime 

 within their own cavities, give strength and solidity to the future 

 protection of the animal. 



The manner in which the skin is removed, as carried on in the 

 lowest forms, is modified only to meet conditions. In the amphipods, 

 which we (Report Brit. Assoc. 1855) have observed in glass jars, the 

 act is not of long duration, and the animal appears not only capable 

 of swimming about until the moment arrives, but has the power, 

 during the moulting, of removing at almost every stage, if it be 

 disturbed, from one place to another, for better security. It grasps 

 with one or more of its anterior feet some fixed weed or secure 

 material as an anchorage ; it then commences to free itself, which 

 appears not to be an act of discomfort, if we may judge from the 

 small amount of trouble with which the operation is conducted. 

 The process appears, as in the crab, to be the result of an internal 

 growth by which the animal has become too large for the skin. It 

 splits at the margin where the dorsal and sternal arches of the three 

 anterior segments of the pereion meet. After some tolerable exertion 

 the posterior portion of the animal, together with its limbs, is with- 

 drawn from its position in the old skin, and then follows the body, and 

 lastly a few more struggles and the head is free, with the entire animal 

 from the old skin. Unless disturbed, the animal, which is extremely 



