THE INFUSORIA 437 



unite usually into more or less conspicuous main ducts so-called 

 " feeding-canals " which empty themselves into a contractile 

 vacuole. 



The arrangement of the contractile vacuoles and canals varies 

 considerably in different species. Thus, in Stentor (Fig. 8) there is 

 a single contractile vacuole, with a feeding-canal running the length 

 of the body ; so also in Spirostomum (Fig. 180, f.c.). In Paramecium 

 (Fig. 185) there are two contractile vacuoles near each end of the 

 body. The vacuole contracts suddenly, diminishing to a tiny 

 globule, and then some six or eight feeding-canals make their 

 appearance, arranged round the vacuole in a star-like figure, but 

 at first distinct from the central vacuole. The inner ends of the 

 feeding-canals gradually swell, and, after reaching a certain size, 

 burst through and empty themselves into the central vacuole, which 

 grows slowly to its full size, and as it does so the feeding-canals 

 disappear by degrees from view. When the vacuole has reached 

 its full size, it empties itself to the exterior, and the process begins 

 again. The contractile vacuole itself may be considered as a cen- 

 tralized portion of the canal-system, and though when full it 

 bulges into the endoplasm, it belongs strictly to the ectoplasm. 



The endoplasm is the seat of nutrition, and also, as containing 

 the nuclear apparatus, of reproductive processes. It is of fluid 

 consistence, and exhibits streaming movements, termed " cyclosis " 

 that is to say, currents of protoplasm which flow round constantly 

 in one direction, as if the endoplasm was being stirred round and 

 round. The endoplasm contains enclosures of various kinds, chief 

 amongst which are the food- vacuoles, containing ingested food- 

 particles in process of digestion. The food- vacuoles are formed at 

 the base of the oesophagus, down which food-particles are wafted 

 by the action of the adoral cilia and membranes. When full-sized, 

 the food- vacuole becomes detached from the end of the oesophagus, 

 like a soap-bubble from a pipe, and passes round the body in the 

 currents of the endoplasm, the indigestible faecal residue being 

 expelled finally from the anal pore (p. 433, supra). In addition to 

 food- vacuoles, the endoplasm contains various metaplastic grains, 

 excretory granules, " spheroplasts " (see p. 448), and sometimes 

 symbiotic algae. 



The nuclei are typically two in number a large, conspicuous 

 macronucleus, staining deeply ; and a micronucleus of much smaller 

 size, often very inconspicuous, and difficult to stain. In primitive 

 forms the macronucleus is a compact body, and the micronucleus 

 appears as a small refringent globule close beside it, often lodged 

 in a depression of the surface of the macronucleus (Fig. 185, N, n). 

 But the nuclei show very great variation in form, number, and 

 appearance. The macronucleus may be drawn out into the shape 



