PHYSIOLOGY OF THE INVERTEBRATA. 29 



externally in a slight knob, the latter being pierced with 

 a small air-hole. These knob-like projections are used for 

 seizing the prey. The protoplasm of the captured infusorium 

 slips slowly through the tentacuki,* and is gathered together 

 in the interior of the Acinda in the forra of small globules. 

 Therefore we have in these tentacula a direct advance on 

 the flagella of the monads and the pseudopodia of the forms 

 already described, inasmuch as the former are not only pre- 

 hensile organs, but act as suckers. 



The Acinctcc possess one or more contractile vacuoles, and 

 in this respect differ from the monads. 



The Infusoria ciliaia are characterised by having number- 

 less cilia (Fig. 3, c). These cilia are either localised to the 

 oral side of the body, or form a zone round it, or are scattered 

 over its external surface. 



In all the Lifusoria ciliata there is an oral region, or mouth, 

 an oesophagus which leads into the central protoplasmic mass, 

 and there is an anal region. In this division of the Tiifiisoria 

 the differentiation into parts has gone so far as to produce a 

 mouth, an oesophagus, and an anal region ; but the alimentary 

 canal is broken, for the oesophagus and anal region do not 

 form one continuous tract. The Infusoria ciliata have con- 

 tractile vacuoles. 



In the Vorticelkc (Fig. 3, c) " the oral region presents a 

 depression, the vestibule from which a permanent oesophageal 

 canal leads into the soft semi-fluid eudosarc, where it ter- 

 minates abruptly ; and immediately beneath the mouth, in 

 the vestibule, there is an anal region, which gives exit to the 

 refuse of digestion, but presents an opening only when fascal 

 matters are passing out." The Vorticellce possess a contractile 

 vacuole as well as several gastric or food vacuoles. The latter 

 are filled with a clear fluid containing the swallowed bodies 

 (algte, &c.). The food vacuoles do not remain stationary, but are 

 conveyed round the inner part of the body, so that the particles 

 of food contained in these vacuoles undergo digestion. 



* See Stein's Dcr Organismm der Infusionsth'e r, vol. i, p. 76. 



