194 THE MECHANICAL FUNCTIONS. 



organs of locomotion. The greater number, indeed, 

 are formed for an existence as completely sta- 

 tionary as the Zoophytes attached to a fixed base. 

 The Oyster, the Muscle, and the Limpet, for ex- 

 ample, are usually adherent to rocks at the bottom 

 of the sea, and are consequently dependent for 

 their nourishment on the supplies of food casually 

 brought within their reach by the waves and cur- 

 rents of the ocean. This permanent attachment 

 to the solid body on which they fix their abode, 

 does not, however, take place till they have arrived 

 at a certain period of their growth : for at the com- 

 mencement of their separate 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 pre- 

 serve 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, Nature 

 has ordained that the purposes of Mechanical sup- 

 port and protection shall be answered by the for- 

 mation 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 may be either univalve, 

 bivalve, or multivalve, according as they consist of 

 one, two, or more pieces. Univalve shells have 



* 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 infusory 

 animalcules, and of the young of polypi. 



