94 ZOOLOGY SECT. 



The form of the body is very varied : it may be ovoid (Fig. 

 72, 7), kidney-shaped (2), trumpet-shaped (3), vase- or cup-shaped 

 (4, 9) ; produced into a long, flexible, neck-like process (5), or into 

 large paired lappets (6); flattened from above downwards, or 

 elongated and divided into segments reminding us of those of 

 a segmented worm (8). 



Most species are free-swimming, but some are attached to 

 weeds, stones, &c., by a stalk. This may be a purely cuticular 

 structure (9), or may contain a prolongation of the cortex in the 

 form of a delicate contractile axial fibre (Figs. 74 and 75, ax. /.), 

 which serves to retract the Infusor, its contraction causing the 

 stalk to coil up into a close spiral. 



The arrangement of the cilia is also subject to great varia- 

 tion, and presents four chief types. In the holotrichous type, of 

 which Paramoecium is an example, the cilia are all small, equal- 

 sized or nearly so, and arranged in longitudinal rows (Fig. 70, Fig. 

 72, 1). The second or heterotrichous type is seen in its simplest 

 form in Nyctotherus (Fig. 72, 2), in which the left side of the 

 peristome is bordered by a row of specially large adoral cilia, the 

 rest of the body being covered with small cilia. In Stentor (3) 

 the peristome is situated on the broad distal end of the trumpet- 

 shaped body, and the adoral band of cilia takes a spiral course. 

 This leads us to the peritrichous type of ciliation : in Vorticella 

 (Fig. 74) the vase-shaped body is, for the most part, quite bare of 

 cilia, but around the thickened edge of the peristome passes one 

 limb of a spiral band of large cilia united at their bases, the other 

 limb being continued round a raised lid-like structure, or disc, into 

 which the distal region is produced. This arrangement of cilia 

 reaches its greatest complexity in Epistylis plicatilis (Fig. 72, 9), 

 in which the ciliary spiral makes no fewer than four turns. 



But it is in the hypotrichous type that the most extraordinary 

 modifications are found. The flattened body bears on its dorsal 

 surface mere vestiges of cilia in the form of very minute processes, 

 while on the ventral surface the cilia take the form of large 

 hooks, fans, bristles, and plates with fringed ends (Fig. 72, 

 7). The hooks and plates do not vibrate rhythmically like 

 ordinary cilia, but are moved as a whole at the will of the animal, 

 thus acting as legs. The hypotrichous Ciliata, in fact, in addition 

 to swimming freely in the water, creep over the surface of weeds, 

 &c., very much after the manner of Woodlice. One of the most 

 extraordinary forms in this group is Diophrys (7), the size and 

 arrangement of its polymorphic cilia giving it a very grotesque 

 appearance. In another genus (10) the distal end of the flask- 

 shaped body bears a circlet of large fringed cilia, giving the animal 

 the appearance of a Rotifer (vide Section VII.). 



In addition to cilia, many genera possess delicate sheets of 

 protoplasm or undulating membranes in connection with the 



