GASTEROPODA. 549 



If, at the same time, it is confined within a naiTow cell, or 

 space, it grows only to such a size as will enable it to move 

 about freely ; thus it is made to adapt itself to the neces- 

 sities of a restricted state of existence. Some youug 

 animals in a narrow glass-cell, at the end of six months, 

 were alive and well, and the cilia retained around the 

 tentacles in constant activity ; whilst other animals of the 

 same brood and age, placed in a situation favourable to 

 growth, attained their full size, and produced young, which 

 grew in three weeks to the size of their elder relations. 



Should any injury occur to the shell, or a portion of it 

 become broken off, the calcareous deposit is quickly resumed, 

 in order to replace the lost part; the cells being apparently 

 only half the size of those originally deposited. This 

 may be thought to aflbrd some proof of the stjitement 

 made by Professor Paget, — " that, as a rule, the reparative 

 power in each perfect species, whether it be higher or 

 lower in the scale, is in an inverse proportion to the 

 amount of change through which it has passed in its 

 development from the embryonic to the perfect state. 

 And the deduction to be made from them is, that the 

 powers for development from the embryo are identical 

 with those exercised for the restoration from injuries ; in 

 otiier words, that the powers are the same by which per- 

 fection is first achieved, and by which, when lost, it is 

 recovered. Indeed, it would almost seem as if the species 

 that have the least means of escape or defence from 

 mutilation were those on which the most ample power of 

 repair has been bestowed, — an admirable instance, if it be 

 only generally true, of the beneficence that has prepared 

 for the welfare of even the least of the living world, with 

 as much care as if they were the sole objects of the Divine 

 regard." 



The primordial cell-wall of the cell does not appear to 

 enter into the formative process of the embryo — the cell- 

 contents alone nourishing the vital blastema of the nucleus. 

 A gradual cycle of progressive development once set up, 

 goes on, until the animal is sufficiently matured to break 

 through the cell-wall and escape from the ova-sac. At 

 the same time, it may be inferred, that all this is in some 

 measure aided by the process of endosmose; and that cer- 



