30 MOLLUSCA. 
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, Zunicata, 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 appa- 
rently helpless creatures. Countless millions of 
Infusorial animalcules sport in the clear water, 
altogether unappreciable by our senses, while vege- 
tables clothed with flinty shells, the Diatomacee 
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, there- 
fore, 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 
