Ciliary Mechanisms 51 



the mouth and out of the body. It is because this mech- 

 anism is so intricate and so beautifully adapted to the 

 environment in which bivalves live, and because few facts 

 concerning it have yet been published, that a short de- 

 scription of it, as it appears in the common food mol- 

 lusks, will be given here. 



But first, brief .mention must be made of the minute 

 food organisms which the animal allows to pass into its 

 mouth. Then it may be noted how it deals with the un- 

 desirable mud particles, also brought by the incoming 

 stream, and how it rids itself of them without checking 

 the flow of water into the body. 



There are reasons for believing that, from the begin- 

 ning, lamellibranchs lived on floating organisms that 

 they were able to take from the water. However that 

 may be, all living forms whether creeping, attached, or 

 burrowed in the bottom, now derive their nourishment 

 entirely from microscopically small organisms brought 

 to the body by the action of the gills. 



The food forms thus captured and consumed, belong, 

 with few exceptions, to a universally distributed group 

 of plants known as diatoms. They are single-celled or- 

 ganisms, but possess some structural modifications of 

 great complexity. The bit of protoplasm constituting 

 the living part of the body is enclosed in a case of pure 

 glass, made of two pieces nearly equal in size, one fitting 

 into the other like a pill-box into its cover. This case is 

 often sculptured with extremely minute and exquisitely 

 regular markings, definite patterns characterizing many 

 species. In outline they vary from circular disks to 

 slender rods (Figure 12). On account of their beauty 

 and variety, they are the stock subjects for exhibition by 

 amateur microscopists. 



