54 



THE MICROSCOPE AND ITS REVELATIONS. 



si tic, the one in the large cells of Vauclieria ( 219), and another in the 

 sphere ofVolvox ( 236). The Wheel-like organs from which the class 

 derives its designation, are most characteristically seen in the common 

 Rotifer (Fig. 310), where they consist of two disk-like lobes or projections 

 of the body, whose margins are fringed with long cilia; and it is the 

 uninterrupted succession of strokes given by these cilia, each row of 

 which nearly returns (as it were) into itself, that gives rise by an optical 

 illusion to the notion of ' wheels.' This arrangement, however, is by no 

 means universal; in fact, it obtains in only a small proportion of the 

 group ; and by far the more general plan is that seen in Fig. 309, in 



FIG. 309. 



FIG. 310. 



Rotifer vulgaris, as seen at A with the wheels 

 drawn-in, and at B with the wheels expanded : 

 a, mouth; b, eye-spots; c, wheels; d, calcar (an- 

 tenna?); e, jaws and teeth; /, alimentary canal; 

 Brachionus pala. g, glandular ( ?) mass enclosing it; h, longitudinal 



muscles; i, i, tubes of water-vascular system; fc, 

 young animal ; I, cloaca. 



which the cilia form one continuous line across the body, being disposed 

 upon the sinuous edges of certain lobes or projections which are borne 

 upon its anterior portion. Some of the chief departures from this plan 

 will be noticed hereafter (453). 



446. The great transparence of the Eotifera permits their general 

 structure to be easily recognized. They have usually an elongated form, 

 similai on the two sides; but this rarely exhibits any traces of segmental 

 division. The body is covered with a double envelope, both layers of 

 which are extremely thin and flexible in some species, whilst in others 

 the outer one seems to possess a horny consistence. In the former case 

 the whole integument is drawn together in a wrinkled manner when the 



