POLYPS AXD 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 larvje, 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- 

 i >rgaiis ; 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- 

 . heel. 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 Leucophi'ys patvla 

 (fig. 1 7 1 ), these organs become visible when 

 the animalcule has been fed with minute 

 particles of carmine diffused through the 

 . 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 



is numerous globular cells, many Y\V. jyj Leucophrys 



of which are distended with colouring patula. 



M 



