In treating of the food of the Mollusca, Dr. Johnston divides them into three classes; first, those which take their food in a liquid form, or suspended in water; secondly, those which are more properly carnivorous; and, thirdly, those which feed on vegetable matter.
Under the first division are comprised all those which have no distinct head, including the three classes, Tunicata, Brachiopoda and Conchifera. None of these have any power of pursuing prey, nor any organs for mastication. Yet any one who has ever examined with a microscope, either the sea-water, which appears to the naked eye pure and simple, or the impalpable sediment which lies upon the bottom, will be at no loss to discover abundant organic matter fitted to supply nutriment to these headless, generally stationary, and apparently helpless creatures. Countless millions of Infusorial animalcules sport in the clear water, altogether unappreciable by our senses, while vegetables clothed with flinty shells, the Diatomaceæœ of botanists, equally numerous and equally minute, crowd the mud on the floor of the sea.
In order that these minute bodies should afford nutriment to the headless Mollusca, a simple but effective contrivance is provided. The currents which ceaselessly play over the breathing organs, produced by the cilia which cover them, not only bring water to be respired, but come charged with the various organic particles, both animal and vegetable, that occur in the vicinity. It is, therefore, merely necessary that the orifice of the stomach, which for convenience sake may be called the mouth, be situated in the course of the currents, and be endowed with the power of
