THE INFUSORIA. 95 



version into a globular or oval germ, which, in some species, 

 is wholly covered with vibratile cilia, while, in others, the cilia 

 are confined to a zone around the middle of the embryo. 

 The germ makes its escape by bursting through the body-wall 

 of its parent. After a short existence (sometimes limited to 

 a few minutes) in the condition of a free-swimming animal- 

 cule, provided with an endoplast and a contractile vacuole, 

 but devoid of a mouth, the characteristic knobbed radiating 

 processes make their aj3pearance, the cilia vanish, and the ani- 

 mal passes into the Acineta state. 



The Acinetce have frequently been observed to conju- 

 gate, the separate individuals becoming completely fused into 

 one and their endoplasts coalescing into the 'single endoplast 

 of the resultant Acineta / but it is not certainly made out 

 whether this process has, or has not, anything to do with the 

 process of the development of ciliated embryos just described. 



(c.) The Ciliata. — The characteristic feature of the Ciliata 

 is, that the outer surface of the body is provided with numer- 

 ous vibratile cilia, which are the organs of prehension and loco- 

 motion. According to the distribution of the cilia, Stein has 

 divided them into the Holotricha, in which the cilia are scat- 

 tered over the whole body, and are of one kind ; the Hetero- 

 tricha, in which the widely-diffused cilia are of different kinds, 

 some larger and some smaller ; the Hypotricha, in which the 

 cilia are confined to the under or oral side of the bodv: and the 







Peritricha, in which they form a zone round the body. The 

 great majority of these animals are asymmetrical. 



In the simplest and smallest Ciliata, the body resembles 

 that of one of the Flagellata in being differentiated merely 

 into an ectosarc and endosarc, with an endoplast and a con- 

 tractile vacuole. In most, if not all cases, however, there 

 is not only an oral region, through which the ingestion of 

 food takes place, but an oesophageal depression leads from 

 this into the endosarc ; and it may be doubted whether, even 

 in the simplest Ciliata, there is not an anal area through 

 which the undigested parts of the food are thrown out. 



The genus Colpoda, which is very common in infusions of 

 hay, is a good example of this low form of ciliated Infuso- 

 rium. It has somewhat the form of a bean flattened on one 

 side, and moves actively about by means of numerous cilia, 

 the longest of which are situated at the interior end of the 

 body. At the posterior end is the contractile vacuole, while 

 a large endoplast lies in the middle, as Stein originally dis- 

 covered. Colpodaz frequently become quiescent, retract their 



