442 THE PROTOZOA 



growth of the central part of the scopula on a prolongation of the body- 

 substance, leaving a peripheral ring of scopular rods surrounding a central 

 protoplasmic cord, which furnishes the contractile muscular stalk. 



The mouth and cytopharynx, whether capable of being closed, as in the 

 Gymnostomata, or permanently open, as in other forms, constitute together 

 a conspicuous organ in the holozoic Ciliata, sometimes showing remarkable 

 adaptations to special modes of feeding. In the gymnostomatous genus- 

 Didinium, the cytopharynx contains a peculiar tongue-like organ, a prolonga- 

 tion of the endoplasm, which shows a longitudinal striation due to the presence- 

 of fine rods " trichites." The tongue ot Didinium is used for capturing prey, 

 consisting chiefly of Paramecium and other Ciliata, and the manner in which 

 it is used recalls the tongue of a chameleon. If the Didinium comes into 

 contact with its prey, the tongue is shot out by a violent contraction of the 

 pharynx, and adheres to the victim, which, according to Thon, is killed in- 

 stantly ; but according to Mast larger Ciliata sometimes escape, and in doing 

 so may break off and carry away the tongue of the Didinium. A Paramecium 

 when thus attacked emits a cloud of trichocysts, but none are discharged by 

 the Didinium. The prey when mastered is drawn into the endoplasm by the 

 retraction of the tongue. The recently-described genera Proboscidium and 

 Gymnozoum (Meunier) possess similar organs. The hymenostomatous genus 

 Pleuronema and allied forms are remarkable for the huge size of the un- 

 dulating membrane (Fig. 27). The animal, after swimming freely for a time,, 

 comes to rest, with its body-cilia sticking out stiff and straight ; the undulating 

 membrane is then protruded from the mouth, and by its active movements- 

 serves to waft food-particles into the pharynx. 



The peristome, or region round the mouth, exhibits a wide range of special 

 adaptations in relation to the function of food-capture, as is apparent from 

 the classification given above. Absent or scarcely developed, as a rule, in 

 the Gymnostomata, in the Hymenostomata it has the form, usually, of a 

 simple groove leading to the mouth (Fig. 185, P.) ; in the Spirigera, on the other 

 hand, it is generally disc-like, bearing the adoral zone which terminates in 

 the mouth ; the extreme type of complication is seen in the Peritricha, where 

 the peristomial disc can be contracted completely over the mouth by means 

 of circular myonemes situated in the margin of the disc like a sphincter 

 (see below), while a central funnel-shaped portion is prolonged inwards, with 

 the mouth at its extremity to form the vestibule, in a manner analogous to 

 the stomodeeum of the Metazoa. The adoral ciliary spiral may consist simply 

 of longer cilia, more powerful than those of the general body-covering, the 

 most primitive condition ; or of transversely-planted, comb-like mem- 

 branellae or " pectinellse," the usual arrangement in Heterotricha and Hypo- 

 tricha ; or of a pair of undulating membranes running parallel to one another 

 in the spiral, as in Peritricha. These various structures, seen in optical section 

 in the living state, have often produced erroneous impressions of bristles, 

 cirri, etc. The adoral spiral varies greatly in extent, and the peristomial 

 region shows numerous modifications which cannot be described or sum- 

 marized briefly ; the reader must be referred to the beautiful descriptions of 

 Schroder (864-867), amongst recent witers. In the remarkable peritrichous 

 form Opercularia (Cochlearia) faurei, the adoral spiral takes five complete 

 turns, running like a screw round a sort of retractile proboscis (Collin, 832). 



The ciliary apparatus and its modifications (Fig. 186) have been the subject 

 of much minute and detailed study ; among recent investigators must be 

 mentioned especially Maier (73) and Schuberg (44). The body-cilia run in 

 rows with a meridional, spiral, or other arrangement ; they arise in depressions 

 of the body-surface which have usually the form of furrows, but in some 

 cases (Paramecium, Frontonia] each cilium arises from the centre of a small 

 depressed area of the surface. In Paramecium the areas are for the most 

 part hexagonal in form, but in places they are rhombic (cf. Khainsky, 170'5). 

 The points of the trichocysts are situated in the angles of the polygons, and 

 also in the broader edges between the areas in each row. 



Each cilium takes origin from a basal granule situated at the level of the 



