NUTRITION PROPER. 



345 



surrounded with hollow tentacles opening into the body- 

 cavity, and in its center is the mouth, opening into the 

 stomach or gullet, the lower end of which opens into the 

 general cavity. The stomach does not hang free, but is 

 held firmly in its place by partitions running from the 

 outer wall to the stomach, and dividing the general 

 cavity into man}' triangular apartments. Many parti- 

 tions do not reach the stomach. Below the stomach the 

 partitions end in free scythelike margins, so that all the 

 triangular apartments are in communication with' one 

 another. 



Process of Digestion. — The food is taken, as in 

 medusae, by thread cells, is put into the mouth and 

 swallowed, and, partly at least, digested in the stomach. 

 Whatever is indigestible is thrown back to the sea 

 through the mouth. The partly digested food is then 

 dropped into the general cavity, its digestion completed, 

 and then circulated by ciliary currents throughout the 

 whole interior cavity even to the extremity of the hol- 

 low tentacles, and directly absorbed and appropriated by 

 the tissues. 



PROTOZOA. 



Here, again, we find a great step downward. Thus far 

 we have found a circulating fluid, although in the ccelen- 

 terates it is not blood but digested food, and circulated 

 not by the mechanical action of a heart, but by ciliary 

 currents. But now we find no circulating fluid of any 

 kind. The food is digested and at once appropriated 

 by the living protoplasm. But even in protozoa we find 

 two grades. In the Infusoria (Fig. 233) the mouth is 

 surrounded by cilia, and particles of all kinds are 

 brought there by ciliary currents. If any are suitable 

 for food they are carried down into the stomach; if 

 not, they are rejected and whirled away by the same 

 current. In Rhizopods there is neither mouth nor stom- 



