230 . INFUSORIA. 



number is in the latter case generally small. In a large and rich 

 group — the Flagellata — there is usually only a single flagellum, or at 

 most but a few. 



It is especially the region round the mouth which in such cases 

 bears the cilia, and this is intelligible when we remember that the 

 cilia act not only as locomotor organs, but also in capturing the food. 

 A little whirlpool is set up by their movements, and the minute 

 bodies suspended in the water are swept into the mouth. For this 

 purpose the oral cilia often have a special arrangement, and a more 

 vigorous development than those on other parts of the body. 



The food, which usually consists of fine granules, becomes agglo- 

 merated into a round ball at the lower part of the mouth, and this is 

 swallowed only when it has attained a certain size. It is of course 

 the adjacent parts of the contractile cortical layer which effect this 

 swallowing, and transfer the morsel through a sort of gullet (of course 

 without special walls, and really only a slit in the parenchyma) into 

 the central substance. Some specimens living in favourable nutritive 

 conditions often have their central substance filled with numerous balls 

 of food. Organic pigments, such as carmine and indigo, are similarly 

 ingested, and were first described in 1777 by Baron von Eussworm- 

 Gleichen in a work on microscopic objects. ^ The food-balls, under the 

 pressure of the surrounding body-substance, glide inwards in more or 

 less regular courses, and here and there coming in contact, they flow 

 together ; and one can follow step by step the various changes which 

 they undergo under the action of the juices mixed up with them, 

 until finally the remnants are expelled by the anus. This lies usually 

 at the posterior end of the body, opposite the mouth, though there are 

 instances in which the anus is near the oral end, and in which the 

 mouth is displaced, and comes to open near the middle of the body. 



When the food does not consist of fine granules, as is the case in 

 some Infusoria, but of larger bodies (plants and animals), the mouth is 

 usually a very wide and extensible cleft, which extends over a con- 

 siderable proportion of the whole body. 



An Alimentary Canal is never present ; nor are the niunerous 

 " stomachs " described by Ehrenberg (hence the name " Polygastrica ") 

 anything more than the food enclosed in vacuoles of the body-substance. 

 What has been previously said of the Ehizopoda holds true of the 

 Infusoria, which only differ in the persistence of the openings for 

 ingestion and expulsion ; and we have already mentioned that these 

 openings are sometimes wanting. So far as we know, however, this 

 occurs only in certain parasitic forms (species of the genus Opalina), 

 which, living continually surrounded by fluid nutriment, require only 

 ^ " MikroBkocischfi Augen- upd Gemutfeergotzungen," 1777. 



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