14 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



what curved course, and ends abruptly within the endosarc. A 

 small area in the vestibule, beneath the disc, is devoid of cuticle, 

 and serving, as it does, for the extrusion of remnants of the food, 

 may be termed the anus. Within the endosarc numerous food- 

 vacuoles can usually be distinguished. 



PHYSIOLOGY. 



Vorticella, like Amoeba, is an unicellular animal, but the single 

 cell of which its body is composed is very much specialized or 

 differentiated, since different parts of it are modified for the per- 

 formance of different functions. 



1. Nutrition, The combined action of the cilia produces a 

 current by which minute organisms and organic particles are 

 carried down to the end of the pharynx, and, together with a 

 small amount of water, forced into the endosarc. There, in food- 

 vacuoles, they slowly pass down one side of the body and up the 

 other, the water being gradually absorbed, and the nutritious 

 parts digested as in Amceba. The undigested remnants are 

 ejected from the anus, which is only visible at the moment of 

 extrusion, and are carried out of the vestibule by the ciliary 

 current. 



Under unfavourable conditions Vorticella sometimes detaches 

 itself from its stalk, and swims away by means of its cilia, 

 becoming re-attached, and developing a new stalk, if a suitable 

 spot is reached. As in Amoeba, unfavourable conditions may also 

 lead to encystment, the body, either whilst attached to its stalk or 

 after separation, becoming rounded, and secreting a protective 

 horny cyst. 



In one species of Vorticella, V. viridis, chlorophyll is diffused throughout 

 its body, which no doubt enables it to live, in part, like a green plant, 

 utilizing carbon dioxide as a source of carbon. 



2. Katabolism. As in Amceba, something akin to a digestive 

 secretion is probably formed by the protoplasm. The same waste- 

 products, namely, carbon dioxide, water, and ammonia compounds, 

 result from the breaking-down of the body, and, as before, the 

 contractile vacuole is probably excretory and respiratory, waste- 

 products passing from it into the vestibule, with which it has 

 been observed to communicate, while, on the other hand, oxygen- 

 ated water may be taken up into it from the exterior. The 



