THE LOWER FORMS OF LIFE. 31 



cilia become gradually changed into a stalk, which, by growth is 

 developed into the singular elastic contractile organ shown in 

 Fig. 28 (h). The exact nature of this stalk has not yet been fully 

 made out. My friend Professor Bennett, of Edinburgh, considers it 

 contains contractile tissue, and that it is merely a prolongation of 

 one of the layers of the bell itself. 



Be this as it may, this stalk is certainly a tube, which contains a 

 thin thread-like band, which I have shown in the figure at i in the 

 form it appears in the interior of the stem. 



Now then, the stalk having grown to its full length, the animal- 

 cule may be considered as mature, and we are able to see through its 

 clear body all that is contained therein (Fig. 28). 



Fig. 28. Diagram of Vorticellaa a, the peristome 6, the disc c, the mouth d, the gullet 

 e, opening in gullet/, contractile chamber g, nucleus h, stalk i, contractile filament, 

 supposed to be the continuation of one of the coats of the animalcule. 



First, however, observe the rim round the distal extremity of the 

 cup (a). This is called (from the two Greek words signifying 

 "about" and " mouth ") the peristome.. Note also that this peri- 

 stome does not carry the cilia. This is done by what is termed the 

 " disc," which is placed within the peristome, and has its outer edge 

 furnished with one or more circlets of cilia, which assume a spiral 

 figure. The mouth (c) is placed in a small space between the disc and 

 the peristome, and the cilia are observed to commence a little to the 

 right of the mouth and, going to the left, run once or twice round 

 the edge of the disc, and then terminate inside the mouth in what 

 is called the vestibulum. 



Thus what appears to be the open mouth of the bell is closed in 

 by the disc, and the animalcule has the power of drawing this disc 

 inwards, when the peristome will contract and cover it in. It is 

 impossible to show the curious arrangement in a drawing, but the 

 above description will, I think, render it tolerably clear, with the 

 aid of the diagrammatic Fig. 28. Well, then, the action of these 

 cilia forms a current, which brings to the mouth other animalcules 



