POLYPS AND INFUSORIA. 



161 



digestion enters the system of the hydra, as no vessels have 

 been discovered in them ; that the colour of the granular 

 parenchyma depends in some measure on the nature of the 

 food is satisfactorily shown ; thus, when a polype feeds upon 

 red larvae, or upon black planarise, the granules acquire a 

 similar hue, although the fluid in which they float remains 

 colourless; these granules move about in the parenchyma of 

 the animal, and give the appearance of globules of blood un- 

 dulating at large through the general tissue of the polype. 

 Should the Hydra be made to fast for a considerable time, 

 the granules lose their colour, and become almost transparent, 

 in a manner similar to that by which the blood-globules of 

 frogs lose their redness during the winter months, when de- 

 prived of nourishment. 



[ 314. The researches of Ehrenberg have demonstrated 

 that the INFUSORIA admit of a natural division into two 

 groups, founded on the degree of development of their diges- 

 tive organs ; the one group comprehends those in the interior 

 of whose bodies numerous cellular globules are seen, into which 

 alimentary matters pass : from the many gastric cavities pos- 

 sessed by these animalcules they are called POLTGASTRICA (fig. 

 171). In the second group we find a more perfect organization ; 

 the mouth is large, opening into an esophagus and stomach, 

 in which are found gastric teeth, a distinct intestine, and anus ; 

 around the head are numerous ball-shaped bodies, furnished 

 with cilia, which perform motions resembling those of a revolv- 

 ing wheel. The group is therefore called ROTIFERA (fig. 172). 

 The structure of the digestive organs of many of the inferior 

 forms of polygastrica is still involved in much obscurity ; but in 

 the higher forms, as in Leucophrys patula 

 (fig. 171), these organs become visible when 

 the animalcule has been fed with minute 

 particles of carmine diffused through the 

 water. The body is covered with long cilia, 

 which form a circle round the mouth, 

 their vibrations causing currents of water 

 to flow therein, together with the minute 

 particles on which Leucophrys subsists ; 

 the intestine is seen taking a winding 

 course through the body, having appended 

 to its walls numerous globular cells, many -pis. 171. Leucophryi 

 of which are distended with colouring patula. 



