26 MECHANICAL FUNCTIONS. 



pieces, as seen by the preceding figure, and of allowing 

 constant additions to be made to the margins of these 

 pieces. In this manner it is obvious that the whole struc- 

 ture would be enlarged without changing the shape. 

 Fig. 19 shows the appearance of these plates when 

 magnified. 



54. By employing his spines as feet, or levers, this animal 

 is capable of making considerable progress along the bot- 

 tom of the sea. 



MOLLUSCA. 



55. The mollusca, as the name signifies, are animals 

 with soft bodies. They have neither bones, nor hard parts 

 corresponding to the bones of the higher orders of animals. 

 This order includes all those animals which live in calcare- 

 ous habitations, constructed by themselves, and so far as 

 they are popularly known, are called shellfish, as the 

 oyster, muscle, and clam. 



56. The shells of the mollusca are formed either of one 

 or of several pieces, which are called valves. Those of 

 one piece are called univalves, those consisting of two 

 pieces are termed bivalves, and those of more than two 

 pieces are multivalves. 



57. This order presents a vast number, and variety of 

 individuals, many of which have been minutely examined, 

 and arranged into species, genera, and orders, forming 

 a distinct object of study, entitled the science of CON- 

 CHOLOGY. It is proper, however, to state, that this sci- 

 ence is not founded on the physiology of the animals 

 which inhabit these shells, but on the forms and pecu- 

 liarities of the shells themselves. This arose, originally, 

 from the necessity of the case, for in a great many in- 

 stances the shell is readily obtained, being cast upon the 

 seashore, empty, while the animal which inhabited it is 

 never seen, because while alive it lived only in the un- 

 fathomable depths of the ocean. But could the classi- 

 fication have been founded on the animal organization, 

 it is obvious that this method would be practicable only 

 to a few, since the animals could not be preserved foi 



What docs the term mollusca signify? What are mollnscus animals? 



