MOLLUSC A. 215 



rate existence, that is, immediately after they 

 are hatched, they are free to move in the water, 

 and to roam in search of a habitation. In this 

 respect, therefore, they preserve an analogy with 

 the gemmules of sponges, and of polypi, which 

 exercise locomotive powers only in the early 

 stages of their developement.* 



The organization of the Mollusca being unfitted 

 for the construction of an internal skeleton, Na- 

 ture has ordained that the purposes of mechani- 

 cal support and protection shall be answered by 

 the formation of hard calcareous coverings, or 

 shells, the result of a peculiar process of animal 

 production. These shells are formed either of 

 one piece, or of several ; the separate pieces, in 

 either case, being termed valves; so that shells 



* This analogy is strengthened by the circumstance that the 

 movements of many of these animals, in the first periods of their 

 existence, are effected by the same mechanism of vibratory cilia 

 which we found to be instrumental in the progression of the infu- 

 sory animalcules, and of the young of polypi. On observing the 

 first evolution of the ova of the Buccinum undatum, Dr. Grant 

 found them to consist of groups of spherical gelatinous bodies, 

 which soon become covered on one side with a transparent enve- 

 lope, the rudiment of the future shell ; while, on the other side, 

 the gelatinous matter is extended outwards, so as to form the 

 margin of an internal cavity, of which the entrance is surrounded 

 with vibratory cilia, and in the interior of which a revolution of 

 particles is seen, indicating a constant current of fluid. The vi- 

 brations of these cilia are perceived long before the pulsations of 

 the heart, and even before any appearance of that organ is visible ; 



