MOLLUSCA. 67 



feet. In few of the- animals which inhabit fixed shells can 

 a foot be observed. They are more simple in their orga- 

 nization than the free shells, and are destitute of absorbing or 

 ejecting syphons, the place of these being supplied by holes 

 in the duplicature of the cloak. This last distinction, how- 

 ever, is not peculiar to the fixed shells, although found in 

 all of them. 



Among the free shells, a very important circumstance 

 occurs, which we have already noticed, viz. that some of 

 these adhere to rocks and stones by means of temporary 

 threads produced by the animal. They are termed byssi- 

 ferce. Independent of the utility of this power of produc- 

 ing threads of attachment, to the economy of the animals, 

 the byssiferae must possess at least three organs of which 

 the other testaceous mollusca are destitute. The first of 

 these is a gland for the secretion of the substance of which 

 the threads are formed ; the second, a foot so constructed 

 as to be capable of spinning these threads and fixing them 

 to the rocks or other bodies to which they are intended to 

 adhere ; and the third is a muscle in the animal to which 

 the inner end of these threads may be attached, and which 

 muscle, in general, has the power of contraction and elon- 

 gation. This character, then, appears perhaps of the very 

 highest order, so that, in a natural arrangement, we might 

 divide the molluscous bivalves into such as spin threads of 

 attachment, and such as do not. We must, however, con- 

 fess, that the byssiferae have scarcely any other subordinate 

 characters in common, to warrant such an arrangement. 



In general, the valves of which the shell consists close 

 upon each other in such a manner as to leave no opening. 

 In a few genera, however, the valves do not close upon 

 each other at one end, and sometimes at both ; the point 



