185 THE MECHANICAL FUNCTIONS. 



on the supplies of food casually brought within their reach 

 by the waves and currents 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 commencement 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 

 preserve an analogy with the gemmules of sponges, and of 

 polypi, which exercise locomotive powers only in the ear- 

 ly stages of their devlopment.* 



The organization of the Mollusca being unfitted for the 

 construction of an internal skeleton. Nature has ordained 

 that the purposes of mechanical 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; 



* 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 mechanismof vibratory cilia which we found to be instrumental 

 in the progression of the infusory animalcules, and of the young of polypi. 

 On observing the first evolution of the ova of the Buccinum undaium. Dr. 

 Grant found them to consist of groups of spherical gelatinous bodies, which 

 soon become covered on one side with a transparent envelope, the rudiment 

 of the future shell; while, on the other side, the gelatinous matter is extend- 

 ed outwards, so as to form the margin of an internal cavity, of which the en- 

 trance is surrounded with vibratory cilia, and in the interior of which a revo- 

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

 tions of these ciha are perceived long before the pulsations of the heart, and 

 even before any appearance of that organ is visible; th(jy are, indeed, the 

 first indications of life in the embryo. The cilia are in activity even before 

 the animal is hatched: for while confined within the q^^, it is seen almost 

 continually revolving around its centre; amotion which appears destined to 

 bring a constant 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 e^g 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 appear- 

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

 lusca, such as the Dwis, Eolis, &c., which have no shell.— Edin. Journal of 

 Science, Vol. vii. 



