THE INFUSORIA 



433 



fected contractile mechanisms, enabling the animal to stretch oat 

 a long way from the base of attachment, or to retract itself close to 

 it. Sedentary forms may also secrete round themselves a protec- 

 tive sheath or tube. 



Structure of the Body. The mouth, or cytostome, is an aperture 

 leading into a longer or shorter oesophagus, or cytopharynx, which 

 ends blindly in the endoplasm. The indigestible remains of the 

 food are cast out through a pore in the cuticle a cell-anus, or 

 cytopyge, which, though a permanent structure, is usually only 

 visible at the moment of defsecation ; but in some cases there is a 

 distinct anal tube leading to an anal pore, visible at all tunes. In 

 the Gymnostomata (see p. 439, infra) the mouth can be closed or 

 opened by a system of rods contained in the wall of the oesophagus 

 (Fig. 184), which contains no vibratile apparatus ; but in all other 



N 



A B C 



FIG. 182. A and B, Euplotea patella : A, ventral view ; B, dorsal view ; C, EupLaUs 

 harpa. In all the figures : N, macronucleus ; n, micronucleus ; c.v., contractile 

 vacuole ; crh, cirri ; p.m., peristomial membranellae ; F, area containing food- 

 vacuoles enclosed by the macronucleus. After Stein, the micronucleus added 

 from original preparations. 



Ciliata the mouth (if present) is permanently open, and the oeso- 

 phagus has no rod-apparatus, but contains one or more undulating 

 membranes. In the orders Heterotricha, Hypotricha, and Peritricha 

 (see pp. 439, 440, infra), the peristome contains a spiral zone of 

 cilia modified in various ways, leading to the mouth, and continuous 

 with the undulating membrane in the oesophagus. In the two first 

 of these orders the cilia in the adoral zone are generally fused in 

 transverse rows to form membranellae. In the Peritricha the adoral 

 zone is composed of two parallel undulating membranes, and in 

 this order the mouth, together with the anus and the contractile 

 vacuoles, are sunk into a funnel-shaped or tubular depression called 

 the " vestibule" (Fig. 183, F.). The two undulating membranes, after 

 describing a spiral which varies from one and a quarter to five com- 



28 



