Chap IV.] 



PARASITIC HABITS. 



177 



The influence of the parasitic mode of life on 

 the organs of digestion is exceedingly well marked. 

 One of the most obvious and common results is the 

 loss by endoparasites of a mouth ; this, among the 

 Protozoa, obtains in the Gregarines, which, living in 

 organs or cavities of other animals (such as the intes- 

 tine of the lobster, or the testicular reservoirs of the 

 earthworm) that are rich in nutrient fluids, obtain 

 their necessary nourishment 

 through their cuticle by the merely 

 physical process of osmosis. 

 Among the ciliated .Infusori an s, 

 Opalina is mouthless. The same 

 phenomenon is seen among the 

 Metazoa in Echinorhynchus and 

 the Cestoda, which in their adult 

 condition live always in the di- 

 gestive cavity of Vertebrates. 

 What is certainly true of Tsenia 

 serrata (Fredericq), namely, that 

 no digestive ferment is to be found 

 in anv part of its body, is doubt- Fig. 79. Atineta tube- 

 less true also of other Cestoda, ^VaextendelZd 

 and is to be explained by the retracted, 

 fact that these animals live in the 

 midst of food which is being made ready to pass 

 through animal membranes. 



In another large set of cases food is obtained by 

 suction; among the Protozoa this is seen in the ecto- 

 parasitic Acinetse (Fig. 79), where elongated tubular 

 processes of protoplasm arise from the surface of 

 the body ; these tentacles, as they are often called, 

 are capable of very rapid protrusion ; their knobbed 

 ends widen into sucking discs, and are able to pene- 

 trate the cuticle of their prey, which are ordinarily 

 ciliated Infusoria ; the semifluid endosarc is then drawn 

 up through the granular axis of the sucking tube. 



M-16 



