CILIATA 97 



of the lower genera this is anterior and terminal, or nearly so (Fig. 88 A), 

 but usually it is removed to one side of the body (Fig. 86 E). This side is 

 then said to be "ventral", and that opposite to it is "dorsal". 



The mouth, either is merely a soft patch of exposed endoplasm or 

 possesses 2. gullet (p. 15). In a relatively few cases (including all those 

 in which the mouth is terminal and a few of those in which it is 

 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, Vorticella). 

 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. 16, 83 A). 

 The higher forms have along the outer edge of the peristome a food- 

 gathering row of cirri or membranellae, the adoral wreath (Fig. 89, 

 ad.mae.). Typically, the spiral is open, as in Paramecium ^ but in 

 some cases, as in Stentor (Figs. 83 B, 88 C), it has contracted, 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. 89, 90). 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 oif the 

 peristome 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 



