ii . PHYLUM PROTOZOA 83 



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

 62, 1), kidney-shaped (2), trumpet-shaped (3), vase or cup-shaped 

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

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

 elongated and divided into segments reminding us of those of an 

 articulated 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. 64 and 65, ax.f.), 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. 60, Fig. 

 62, 1). The second or Jietcrotrichous type is seen in its simplest 

 form in Nyctotherus (Fig. 62, 2), in which the left side of the 

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

 rest of the body being covered with small cilia. ; In Stcntor (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 peritriclwus type of ciliation : in Vorticella 

 (Fig. 64) 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, 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. 62, 9), in which the ciliary 

 spiral makes no fewer than four turns. 



But it is in the liypotricJious 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 

 of the cuticle, while on the ventral surface the cilia take the form 

 of large hooks, fans, bristles, and plates with fringed ends (Fig. 62, 

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

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

 thus acting as legs. The heterotrichous 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 Dioplirys, the size and arrange- 

 ment of its polymorphic cilia giving it a very grotesque appear- 

 ance. 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 infra, Section VII.). 



In addition to cilia, many genera possess delicate sheets of 

 protoplasm or undulating membranes in connection with the 



