98 THE INVERTEBRATA 



twisted in the opposite direction, makes this view impossible. The 

 origin of the Peritricha may be explained as follows (Fig. 83). In 

 hypotrichous forms which had taken to fixing themselves to the sub- 

 stratum by that (ventral) side which they applied to it, the mouth, 

 being no longer of use in its ventral situation, moved to the left side. 

 The peristome accordingly came to run along the edge of the body, 

 around which it became continued on the dorsal surface. In dorsal 

 aspect its direction is of course reversed. The body, in correspondence 

 with the changed habit of life, has shortened, till its outline, seen from 

 above, is circular, and has deepened. Thus the oral-aboral axis of the 

 Peritricha is not anteroposterior as in Stentor, but dorsoventral. 



Fig. 83. A diagram of the disposition of the peristome in various ciliata. 

 A, Ventral view of a typical heterotrichous form. B, Similar view of Stentor. 

 C, Ventral (aboral) view of a peritrichous form without stalk, such as Tricho- 

 dina (Fig. 86 D). C, Dorsal (oral) view of the same. ad. adoral wreath of 

 membranellae ; vest, vestibule ; ^er. peristome. 



The general surface of the body is in the lower and in some of the 

 higher genera uniformly covered with cilia, but most of the more 

 highly organized forms are naked save where there stand certain 

 special pieces of ciliary apparatus. The ectoplasm (Fig. 84) has a 

 definite and often complicated structure. There is always a tough 

 pellicle, which is frequently sculptured. Under it is often an alveo- 

 lar layer of minute, regular vacuoles. When there are myonemes, 

 these lie on the inner walls of larger canal vacuoles of this layer. Under 

 it again is usually a layer, the cortex^ whose firm consistency prevents 

 the granules, vacuoles, etc., of the endoplasm from entering it, though 

 it may possess small granules of its own. The basal granules of the 

 cilia lie immediately below the alveolar layer; trichocysts are im- 



I 



