STRUCTURE OP THE CELL OP ACEPHALA. 71 



is not the least curious part of their history. They 

 subsist, for the most part, like vegetables, without the 

 trouble of seeking for prey. It is brought to the door 

 of their shells, and they have but to " gape and swallow 

 it." The water which enters at the openings in the 

 mantle brings in with it nourishing particles of one 

 kind or other, minute animals, &c. These, floating about 

 in the shell, come under the influence of millions of 

 minute cilia or vibratory hairs which clothe every part 

 of the branchial fringe, and which, by their constant 

 motion, form a current strong enough to drive forward 

 to the mouth whatever is floating in the water. The food 

 is thus presented to the lips, which have only to decide 

 whether to receive it or let it pass into the influence of 

 the retreating current, which will carry it out of the shell. 

 To so low a type is animal will reduced in these passion- 

 less creatures, which, nevertheless, exhibit the most won- 

 derful perfection in the construction of their minutest 

 organs, and the most beautiful adaptations of means 

 to ends. The beauty of the shells of many of them is 

 apparent to all the graceful forms of many species of 

 Venus and Chione the rich colouring of the Pectens, 

 the Spondyli, and Tellince but all these beauties are 

 less impressive to the mind than the exquisite structure 

 of the mantle by which these shells are secreted, and 

 the admirable order with which the very particles of 

 the shells are ai'ranged ; an order so exact, that the 

 species to which a minute fragment of a shell belongs 

 may often be determined, or approximated to, by mak- 

 ing a microscopic examination of thinly-cut slices. 

 Thus, an examination of shelly particles, no bigger than 



