PHYLUM PROTOZOA 



95 



obviously structures of the same character as trichocysts, and 

 their resemblance to the ncmatocysts so characteristic of Coelenterata 

 {vide Section IV.) is singularly close. 



Digestive Apparatus. — Many parasitic forms (Fig. 71, 8, 17 ; 

 Fig. 75) have no mouth or gullet, and are nourished by absorption 

 of the digested food in the intestine of their host. The simplest 

 condition of the ingestive apparatus is found in Prorodon (Fig. 

 71, 1) and its allies, in which the mouth (7)ith.) is at one pole 

 of the ovoid body, and is closed except during the ingestion of 

 food, and the gullet (</.) is a short, straight tube. Such forms, 

 on account of the synametrical disposition of their organs and the 

 want of differentiation of their cilia — they are all holotrichous— 

 may be considered as the lowest or least specialised of the Ciliata. 



I.Dicfy ocystQ 



3.Thunicola ^.Optirydium 5. S M chof richo 

 2. Pyxicolo 



Fig. 72. — Various forms of Ciliata. In 1 the shell alone is shown ; m, contractile fibre ; op. 

 operculum. (From Butschli's Protozoa, after various authors.) 



From them there is a fairly complete gradation to genera, like 

 Paramoecium, having the permanently open mouth on the left side 

 of the ventral surface, at the end of a well-marked buccal grove 

 or peristome. Vorticella (Fig. 73) and its allies are peculiar in 

 having the edge of the peristome (per.) thickened so as to form a 

 j)rojecting rim, and in the development of an elevated disc (d.) from 

 the area thus enclosed: the mouth (mtJt.) lies between the peri- 

 stome and the disc, and between it and the gullet proper (gull.) is 

 interposed a section of the ingestive tube called the vestibule 

 into which the reservoir opens, and which contains the anal 

 spot. In Nyctotherus (Fig. 71, 2) and some other genera there is, 

 instead of the temporary anal spot described in Paramoecium, a 

 distinct anal aperture (a.). 



