208 COMPARATIVE ANATOMY 



fibrils at the hinder extremity of the body are easily recog- 

 nisable, whilst those at the anterior end are difficult to see. 



Anyone who watches a living Vorticella will easily see that 

 the creature not only springs to and fro by the contractions of 

 its stalk, but that it also has the power of expanding and 

 unfolding its peristomial disc. When it is fully expanded the 

 peristomial rim is everted, somewhat like the mouth of a bell, 

 the disc is protruded, and the beautiful whorls of cilia situated 

 on these organs are fully displayed. In some species the disc 

 is protruded far above the margin of the peristome ; in others, 

 as in V. monilata, it hardly rises to the level of the peristomial 

 rim. 



In contraction the disc is withdrawn and the peristomial 

 ring is contracted and folded inwards over it. The whorls of 

 cilia are turned inwards and covered over by the infolded 

 margins of the peristomial rim, and the whole body assumes the 

 shape of a pear. It has been said that the contraction of the 

 peristome is effected by means of a circular layer of contractile 

 fibres or sphincter lying in the peristomial ring. Such fibres 

 may be present one can hardly imagine how the contraction 

 of the peristome could take place without them but many of 

 the best observers have failed to distinguish them, and the 

 writer has been equally unsuccessful. 



The contractile filament, formed, as has been shown, by the 

 union of the basal contractile fibres of the disc, traverses the 

 stalk as a homogeneous doubly refracting thread, which in the 

 expanded condition forms a very loose spiral, in the contracted 

 state is tightly coiled up. The filament is ensheathed in a 

 transparent elastic tube which is not a continuation of the so- 

 called cuticle of the body, but an ectoplastic product secreted 

 by the posterior end of the animal, and probably of a chitinous 

 nature. The space between this tube and the contractile 

 filament is occupied by a transparent homogeneous mass of 

 gelatinous consistency, and it is probably due to the presence 

 of this substance that on contraction of the filament the whole 

 stalk is coiled up in a spiral, and the body drawn down towards 

 the point of attachment. The stalk being closed below, the 

 contractile filament does not project beyond it. 



Returning to the structure of the body, we find that the 

 vestibule is continued into a narrowed ciliated gullet, which, 

 in V, mpnilata, turns through a half-spiral and ends in the 



