32 General Morphology of the Protozoa 



with smaller vacuoles. In fact, growth of the vacuole in spurts, instead 

 of a steady increase in volume, suggests that fluid enters the contractile 

 vacuole of Amoeba proteus almost entirely through the fusion with ac- 

 cessory vacuoles (161). In Eudiplodinhim, however, growth of the vacuole 

 in late diastole apparently is not dependent upon accessory vacuoles, 

 and the vacuolar membrane presumably is responsible for the segrega- 

 tion of fluid (161). 



Systole usually involves discharge of the contents directly to the out- 

 side of the body, but there are exceptions. The vacuoles of Euglenida 

 empty into the reservoir ("gullet"); those of Epistylis and related ciliates 

 (55) empty into the pharynx, often termed the "vestibule" in view of its 

 several functions. The point of discharge is often a differentiated pore as 

 in Parnmeciutn (119) and the Ophryoscolecidae (154). Except during 

 systole, this pore is sealed by a membrane (Fig. 1. 16, B, F) apparently 

 derived from the wall of the preceding vacuole. 



A long contractile tube (Fig. 1. 16, A), instead of a contractile vacuole, 

 extends throughout most of the body between the endoplasm and dorsal 

 ectoplasm in Haptophrya (160). This tube is an apparently permanent 

 structure which is divided in binary fission. Excretory canals extend to 

 dorsal pores, which vary in number with length of the ciliate. Systole 

 involves one or more waves of contraction, but the wall of the tube does 

 not collapse completely, and does not disappear. After systole, small 

 vacuoles appear in the wall of the tube and then fuse to form a continuous 

 lumen. 



Sensory vacuoles 



A supposedly sensory vacuole, located anteriorly under a pellicular 

 cap (Fig. 1. 16, C), occurs in parasitic ciliates belonging to the families 

 Biitschliidae and Paraisotrichidae (46). Fibrils on the wall of the vacuole 

 converge toward the pellicular cap, while the vacuolar cavity contains a 

 number of granules ("statoliths"). Superficially similar vacuoles (Fig. 1. 

 16, D, E), forming a row near the aboral surface of the body, have been 

 described in Loxodes. These vacuoles ("Miiller's vesicles") have been in- 

 terpreted both as statocysts (179) and as "excretion-vacuoles" (187). 



Vacuoles in flotation 



Cytoplasmic vacuoles may play an important part in flotation. In 

 Radiolarida (Chapter V), the foamy outer cytoplasm (calymma) is filled 

 with vacuoles which maintain the organisms at a particular depth in the 

 ocean. Under appropriate stimulation, collapse of the vacuoles and re- 

 traction of pseudopodia increase the specific gravity and the organisms 

 sink. When new vacuoles develop in the calymma, the organisms rise 

 again. An analogous phenomenon has been described in Arcella (10). 

 The appearance of gas bubbles in the peripheral cytoplasm, supposedly 



