TESTACEA. 17 



tection. Thence does the shell originate in the ovum, or it may be coeval 

 with the germ itself ; and it accompanies the increment of the animal 

 until liberated by birth. Nor does its tenuity expose the structure de- 

 signed as a protection for the tenant, which affords the most interesting 

 view of any forms employed for the mysterious office of sustaining life 

 and advancing increment. 



But the originating shell is not an exact type of its perfect and 

 adult state : it is often more rude and shapeless. Herein it corresponds 

 with the originating animal. Both are gradually refined into stronger 

 resemblance of the product as matured by age. 



The covering is progressively consolidated by new depositions, and 

 it loses its early transparency, which has been so useful to the observer. 

 New accessions augment the obscurity, and at length interception of this 

 product with a superabundance of calcareous matter, altogether intercepts 

 the view of the animal. 



The safety of the animal is at last entirely dependent on the integrity 

 of the shell. But it can repair many fractures by new secretions, though 

 incapable of a total secretion to protect the body, if entirely deprived of 

 the shell. 



The greatest variety prevails, first in the form of shells ; secondly, 

 in the quantity of matter covering their form. But very little variety 

 prevails among the testaceous animals themselves. I believe that I may 

 affirm, that by far the greater part of shells belongs to an animal related 

 to the Limax, or identically a Limax, and to animals with strong analo- 

 gies to the Ascidia. Besides these, some others are found to be occupants 

 of shells, whose history is less explicit. Where the position of genera or 

 species is so much a matter of opinion, alteration and great modifications 

 may be expected. It seems to be the same with the Testacea as with the 

 Zoophytes : the animals inhabiting each exhibit little disparity, while 

 their dwellings are distinguished by a remarkable variety of formation. 



The multiplicity of inorganic infinitely exceeds the variety of organic 

 forms connecting the two. 



Among the animals so definitely recognised as pertaining to the 

 Testacea, are some which we have distinguished as approaching, 1. tlie 



c 



