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swimming freely through the water. They are met with also in 

 gutters on the house-top, in water-butts, on wet moss, grass, and 

 liver- worts, in the interior of Volvox ylobator and Vancheria, in vege- 

 table infusions, on the backs of Entomostraca, in the viscera of slugs, 

 earth-worms, and Naiades, and in the body-cavities of Synaptce 

 in fact, in almost every place where there are moisture and food. 

 The wheel-like organs from which the class derives its designation 

 are most characteristically seen in the common Rotifer (fig. 600), 

 where they consist of two disc-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.' The disposition of the cilia varies much 

 in the different genera, but it may be said broadly that they are ar- 

 ranged so as to fulfil three different purposes, viz. to bring food to the 

 mouth, to conduct it through the alimentary canal, and to enable the 

 animal to swim. 



The great transparence of the Rotifera permits their general 

 structure to be easily recognised. They have usually an elongated 

 form, similar on the two sides ; but this rarely exhibits any traces of 

 segments! division. The body is covered with an envelope of two 

 layers. The inner of these is a soft lining to the outer, which may 

 be soft and flexible, or membranous and of very varying degrees of 

 stiffness, or even of an inflexible substance capable of resisting the 

 action of caustic potash. In this latter condition it is called a lorica. 

 The greater number of the Rotifera have an organ of attachment 

 at the posterior extremity of the body, which is usually prolonged 

 into a tail or false foot, by which they can affix themselves to any 

 solid object ; and this is their ordinary position when keeping their 

 ' wheels ' in action for a supply of food or of water ; they have no 

 difficulty, however, in letting go their hold and moving through the 

 water in search of a new attachment, and may therefore be con- 

 sidered as perfectly free. The sessile species, in their adult stage, 

 on the other hand, remain attached by the posterior extremity to the 

 spot on which they have at first fixed themselves, and their cilia are 

 consequently employed for no other purpose than that of creating 

 currents in the surrounding water. In considering the internal struc- 

 ture of Rotifera we shall take as its type the arrangement which 

 it presents in Brachionus rubens (fig. 601), a common large and 

 handsome animal, and one that bears the temporary captivity of 

 a compressorium remarkably well. 



Its vase-shaped lorica is hard and transparent ; open in front to 

 allow the protrusion of the head, and closed behind, except where a 

 small aperture permits the passage of the foot. The anterior 

 dorsal edge bears six sharp spines, and the ventral edge has a 

 wavy outline. The head is shaped like a truncated cone, with the 

 larger end forward, is rounded at each side, and carries on its front 

 surface three protuberances (sp). covered with stout vibrating hairs 

 called styles. All round the rim of the head runs a row of cilia which 

 on the ventral surface dips down into either side of a ciliated buccal 

 funnel. At the bottom of the buccal funnel is the mastax (mx), a 



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