216 THE MECHANICAL FUNCTIONS. 



may be either univalve, bivalve, or multivalve, 

 according as they consist of one, two, or more 

 pieces. Univalve shells have generally more or 

 less of a spiral form, and are then called tmhin- 

 ated shells. In a few, the cavity of the shell is 

 divided by transverse partitions into numerous 

 compartments. Some Mollusca have internal 

 shells for the defence and support of particular 

 organs ; and others have shells which are partly 

 external, and partly internal. As respects their 

 shape, colour, and appearance, shells admit of 

 infinite diversity ; yet, as will presently be shown, 

 all are composed of the same kind of material ; 

 and their production and increase are regulated 

 by the same uniform laws. 



they are, indeed, the first indications of life in the embryo. TIic 

 cilia are in activity even before the animal is hatched ; for while 

 confined within the egg, it is seen almost continually revolving 

 round its centre ; a motion which appears destined to bring a con- 

 stant supply and renewal of sea water into the interior of the or- 

 ganization, in order to perfect the formation of the shell before the 

 animal is, as it were, launched into the ocean. Possibly, also, the 

 continued friction of the cilia against the interior of the egg may 

 tend to abrade it, and open a passage for the young animal. No 

 sooner has the animal effected its escape, than it darts rapidly 

 forwards by the motion of its cilia. The same appearances have 

 also been observed by Dr. Grant in the young of different Mol- 

 lusca, such as the Doris, Eolis, &c. which have no shell. — Edin. 

 Journal of Science, Vol. vii. 



