TESTACEA. dl 



by a broader diftusion. These enlarging diffusions are arranged after a 

 particular disposition, according to the species of the animal, so that the 

 cables anchoring the unwieldy Modiolus, stretch in lines quite different 

 from the more slender cordage whereby the common edible Mussel seeks 

 security. 



However, this faculty is enjoyed by only a few of the bivalves : its 

 benefit is hardly evident in some, nor do the great majority possess any 

 such faculty in as far as can be discovered. 



All the animal secretions are extraordinary. Those with which we 

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

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

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

 fulfilling their wonted and proper purpose. 



Secretion of the substance forming the brjssns is undoubtedly the 

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

 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 the 

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

 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 refines 

 it into due proportion when free and strengthened by existence. 



Excellent opportunities of discovering the peculiar organization 

 maintaining the vital function,*, are afforded by its transparence concomi- 

 tant on the earliest stages. 



Certain of our domestic sliells, particularly the univalves, are almost 

 as fragile as those of the smaller Ijirds' eggs. That of the Phym fonthmlh, 

 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 



