PEOTOZOA. 



85 



rouudiug region (peristoma) has often also a special form. A 

 tubular portion, or pharynx (Fig. 28, h) often passes from the mouth 

 into the parenchyma of the body, and from 

 it the ingested morsel finds its way into the 

 soft su.bstance of the latter. 



The position and form of the mouth varies 

 greatly. In many cases it can only be made 

 out during the ingestion of food (as in Amphi- 

 leptus, Loxophyllum), and disappears as soon 

 as the morsel has passed into the parenchyma. 

 There is sometimes an investment of cilia on 

 the tubular pharynx (Paramecium aurelia 

 and bursaria) ; or an undulating membrane 

 (Bursaria flava) ; or a covering of rod-shaped 

 denticles, or fine longitudinal ridges. 



Porodou, Chilodon, Nassula, etc.^ have an 

 investment of small rods in the pharynx, 

 arranged in eel-pot form. A regular thicken- 

 ing- of its walls has been observed in Ervilia 

 and Liosiphon. 



The general presence of an anal opening 

 is not by any means established. There is 

 only in some few cases a permanent and dis- 

 tinctly-marked opening; it can genei'ally be distinguished during 

 the expulsion of undigested food only. This anal spot is as a 

 rule at the posterior end of the body, but is, on the whole, very 

 changeable. It may even appear at the anterior end of the body ; 

 thus, in Stentor it lies near the mouth, and in the Vorticellinte 

 and Ophrydias in the vestibule. Taken on the whole it appears 

 to consist more in the localisation of a function than in the develop- 

 ment of an organ. The products of excretion pass through the 

 differentiated cortical layer of the body at a certain spot, without 

 there being any special organisation of that spot. 



Pig. 28. Diagram of 

 the digestive cavity of 

 Paramseciiim. a Body- 

 space filled vsath soft pro- 

 toplasm, into which the 

 food is taken, h Mouth, 

 c Anns. d Contractile 

 vesicles (after Lach- 

 mann). 



§ 68. 



in all Protozoa the outermost layer of the body has a respira- 

 tory significance, for it is by it alone that an exchange of gases 

 with the surrounding medium can be effected. This relation must 

 also be borne in mind in reference to the increase of surface, which 

 is due to the pseudopodia. The cilia of the Infusoria are of import- 

 ance in changing the water. 



More definite respiratory arrangements are seen when, as in 

 many Protozoa, water is taken into the body. Cavities, which are 

 filled with a fluid, and which gradually contract and completely 

 empty themselves, after having reached their maximum of distension, 

 appear within the protoplasm ; when empty they seeni to disappear. 

 These vacuoles, like the vacuoles in the cells of certain tissues, are 

 partly variable structures, now appearing and now disappearing, and 



