TESTACKA. 61 



by a broader diffusion. These enlarging diffusions arc arranged after a 

 particular disposition, according to the species of the animal, co that tin- 

 cables anchoring the unwirl.ly M<,tlil,tx, stretch in lines quite different 

 from tin- lum-i- .-li-nder cordage whereby the common edible Mussel mk* 

 security. 



However, this faculty is enjoyed by only a few of the bivalven : it* 

 benefit Is hardly evident in some, nor do the great majority pomem anv 

 such faculty in as far as can be discovered. 



All the animal secretions are extraordinary. Those with which we 

 are most familiar are involuntary, following the course of Nature by hid- 

 den means, but some are vitiated, in a manner leading us to believe that 

 tie organs appointed for that office are greatly affected, and incapable of 

 fulfilling their wonted and proper purpose. 



Secretion of the substance forming the bystiu is undoubtedly the 

 result of the faculty spontaneously exercised at the moment, if then 

 be no reservoir for accumulation. If frequent, the resources certainly 

 fail, and the animal becoming exhausted, the shell remains loose. 



A faculty, apparently less spontaneous, but universally diffused 

 among the bivalves, Is the secretion of the calcareous matter composing 

 their shells. 



This secretion seems to originate with the embryonic state, as tin- 

 spark of life is elicited. Its increase accompanies the evolution of tin- 

 living being to more perfect form, and it is beheld in the nascent animal 

 on escaping from the ovum. Then it is less symmetrical, but time refine* 

 it into due proportion when free and strengthened by exi-t<-inv. 



Excellent opportunities of discovering the peculiar organization 

 maintaining the vital functions, are afforded by it* transparence concomi- 

 tant on the earliest stages. 



Certain of our domestic sheik particularly the univalves, are almost 

 as fragile as those of the smaller birds' eggs. That of the Physa fonlinali*, 

 from which some rapacious leech may have extracted the contents, soon 

 appears of the purest white, and almost as thin as tissue paper. Others, 

 proportionally solid, seem capable of resisting external violence. The 

 lower valve of a few of the bivalves is so fragile that I have never seen 



