SURVEY OF UNICELLULAR ANIMALS 



55 



Contractile vacuole, 

 with canals 



Macronucleus 



Micronuclei 



Gastric 

 vacuole 



Contractile 

 vacuole 



Cilia 



Peristome 



Mouth 



Gullet 



astric vacuole 



together constitute the nucleus of the cell, and represent a sort of 

 physiological division of labor of the chromatin complex. 



But it is in the cytoplasm that specialization is most conspicuous. 

 Not only are there general differentiations into ectoplasm and 

 endoplasm, but these regions also have local specializations such 

 as thousands of hair-like, 

 vibratile cilia for loco- 

 motion and securing 

 food, trichocysts for 

 defense, peristome, 

 mouth, and gullet for 

 the intake of solid food, 



GASTRIC VACUOLES for 



digestion, and contrac- 

 tile vacuoles for ex- 

 cretion. And withal, re- 

 cent investigations indi- 

 cate that various parts 

 of the cell may be coor- 

 dinated by a NEUROMO- 

 TOR apparatus. (Figs. 27, 

 135, 225, 226.) 



Paramecium, under 

 favorable conditions, 

 grows rapidly and, when it has attained the size limit character- 

 istic of the species, cell division takes place. This process of 

 multiplying by dividing can go on indefinitely under favorable 

 environmental conditions. But periodically Paramecium under- 

 goes an internal nuclear reorganization process (endomixis). Also 

 now and then individuals temporarily fuse in pairs and inter- 

 change nuclear material (conjugation) — an expression of funda- 

 mental sex phenomena, involving fertilization, which we shall 

 have occasion to consider later. (Figs. 28, 170, 171.) 



Indeed the Infusoria seem, so to speak, to have made the most 

 of their unicellular plan of structure, for Paramecium is fairly 

 representative: it is not the most simple nor yet the most complex. 

 Specialization of one part and another of the cell has produced in 

 the Infusoria a group of animals that, judged by distribution and 

 numbers, is highly successful in the microscopic world of life. 



Trichocysts 



Fig. 27. — Paramecium anrelia. 



