104 THE INVERTEBRATA 



ventral) the mouth is at the surface of the body : in such cases the 

 gullet, if there be one, is an oesophagus, excavated in the endoplasm 

 and capable of being opened and closed to seize the prey which is 

 of some size. Most often, however, there is a vestibule. This, to which 

 also the name '* gullet" is often applied, is a depression leading to the 

 mouth, incapable of being closed, lined by inturned ectoplasm, and 

 containing a ciliary apparatus, which usually includes one or more 

 undulating membranes. By this apparatus the minute objects which 

 constitute the food of all ciliates that have a vestibule are drawn in, 

 being meanwhile, in some cases at least, entangled by a mucous 

 secretion. At the bottom of the vestibule lies the true mouth; some- 

 times an oesophagus is present (Stentor) or is represented by a cleft in 

 the endoplasm (Paramecium). The inner part of the vestibule may be 

 free from cilia, and so simulate an oesophagus {Paramecium, Vorti- 

 cella).^ The vestibule is usually approached by 2i peristome. This is a 

 groove, of varying dimensions, which leads from the front end along 

 the ventral side to the opening (cytostome) of the gullet. It is not 

 straight, but runs in a longer or shorter spiral round the body, so 

 that the anterior end of the latter is spirally deformed (Figs. i6, 84 A). 

 The higher forms have along what is primarily the outer edge of the 

 peristome a food-gathering row of cirri or membranellae, the adoral 

 wreath (Fig. 90, ad.mae.). Typically, the spiral is open, as in Para- 

 mecium, but in some cases, as in Stentor (Figs. 84B, 89 C), it has con- 

 tracted, so that it lies coiled as a crown at the anterior end. In such 

 cases the animal is usually fixed temporarily or permanently by the 

 opposite end. 



The members of one order (Hypotricha) are depressed dorso- 

 ventrally, and have a flat ventral side, along which the peristome runs 

 and which is usually provided with a complex apparatus of cirri 

 (Figs. 90, 91). The animal applies this side to the substratum, in 

 locomotion upon which certain of the cirri are used. The dorsal side 

 is naked save for a few " sensory " cilia. It is probably from such forms 

 that the familiar bell-animalcules and their relations (Peritricha) are 

 derived. In these, the shape of the body and the position of the peri- 

 stome at first suggest that the morphological peculiarities of the 

 group are due to an evolution similar to that by which such forms 

 as Stentor came into being— but the fact that the peristome, which in 

 all other ciliates that- possess it curves clockwise, is in the Peritricha 

 twisted in the opposite direction, makes this view impossible. The 

 origin of the Peritricha may be explained as follows (Fig. 84). In 



^ It is possible that this is a true oesophagus. Other regions are sometimes 

 to be distinguished in the vestibule: in Paramecium, for instance, its outer 

 section has trichocysts and cilia but no membranes, its middle section 

 membranes only. 



