GENERAL BIOLOGY. 21 



UNICELLULAR ANIMALS WITH DIFFERENTIATION 

 OF STRUCTURE. 



The BELii-ANiMALCfULE {Vorticella). 



Amoeba is an example of a one-celled anunal with little per- 

 ceptible differentiation of structure or corresponding division 

 of pbysiological labor. This is not, however, the case with all 

 unicellular animals, and we proceed to study one of these with 

 considerable development of both. The Bell - animalcule is 

 found in both fresh and salt water, either single or in groups. 

 It is anchored to some object by a rope-like stalk of clear pro- 

 toplasm, that has a spiral appearance when contracted ; and 

 which, with a certain degree of regularity, shortens and length 

 ens alternately, suggesting that more definite movement (con- 

 traction) of the form of protoplasm known as muscle, to be 

 studied later. 



The body of the creature is bell-shaped, hence its name ; the 

 bell being provided with a thick everted lip {peristome), covered 

 with bristle-like extensions of the protoplasm (fiilia), which are 

 in almost constant rhythmical motion. Covering the mouth of 

 the bell is a Ud, attached by a hinge of protoplasm to the body, 

 which may be raised or lowered. A wide, funnel-like depres- 

 sion (oesophagus) leads into the softer substance within which 

 it ends blindly. The outer part of the animal (cuticula) is 

 denser and more transparent than any other part of the whole 

 creature ; next to this is a portion more granular and of inter- 

 mediate transparency between the external and innermost por- 

 tions (cortical layer). Below the disk is a space (contractile 

 vesicle) filled with a thin, clear fluid, which may be seen to en- 

 large slowly, and then to collapse suddenly. When the Vorti- 

 cella is feeding, these vesicles may contain food-particles, and 

 in the former, apparently, digestion goes on. Such food vacu- 

 oles (vesicles) may circulate up one side of the body of the ani- 

 mal and down the other. Their exact significance is not known, 

 but it would appear as if digestion went on within them ; and 

 possibly the clear fluid with which they are filled may be a spe- 

 cial secretion with solvent action on food. 



Situated somewhat centrally is a horseshoe-shaped body, with 

 well-defined edges, which stains more readily than the rest of the 

 cell, indicating a different chemical composition ; and, from the 

 prominent part it takes in the reproductive and other functions 

 of the creature, it may be considered the nucleus (endoplast). 



