i8o MANUAL OF ZOOLOGY SECT. 



movements like those of a leech, the oral end and the extremity of the 

 tail being alternately attached. In Rotifers, which are permanently 

 fixed, attachment is effected through the intermediation of the tail, 

 which is drawn out to form a long narrow stalk. In others the tail is 

 absent, or represented only by a pair of ciliated processes. 



The trunk is in some Rotifers enclosed in a glassy cuirass or lorica 

 formed of a thickening of the cuticle. One remarkable form Pedalion 

 has six hollow appendages terminated by feathered setae : and a few 

 other forms are provided with simple or fringed setae. 



The stalked forms inhabit tubes into which the animal can completely 

 retract itself, the substance of the tube being either a delicate gelatinous 

 material, or composed of pellets of mud, or of the animal's faeces. 



The structure of the internal organs is simple. The alimentary 

 canal usually terminates in an anal aperture (a). There is a large 

 pharynx (//) containing a masticatory apparatus, the mastax, usually 

 consisting of three chitinous pieces, or jaws, of complicated form. The 

 nervous system consists of a single ganglion (&r), situated towards the 

 oral end ; and there are usually one or several very simple eyes (<?). In 

 close relation to the brain are one or several processes, the tactile rods 

 (d. /), tipped with non-motile cilia, connected with the ganglion by 

 means of nerves. A pair of longitudinal excretory vessels (w//z), pro- 

 vided at intervals with short branches terminating in flame-cells, usually 

 open into a contractile vesicle which discharges into the terminal part 

 of the intestine. 



The males differ greatly from the females, being nearly always much 

 smaller and degenerate in structure. Three kinds of eggs are produced : 

 large and small summer eggs which always develop without fertilisation 

 {parthenogenesis) and thick-shelled winter eggs, which probably require 

 to be fertilised. 



A few Rotifers live in the sea, but the majority are fresh-water forms, 

 occuring in lakes, streams, ponds, and even in puddles the water of 

 which is rendered foul and opaque by mud and sewage. Frequently 

 the water in which they live is dried up, and the thick-shelled winter 

 eggs may then be widely dispersed by wind. It is even stated that the 

 adult animals may survive prolonged desiccation and resume active life 

 when again placed in water. Many forms cling to the bodies of higher 

 animals in order to obtain a share of their food, thus leading a sort 

 of commensal existence. Others go a step further and become true 

 external or internal parasites. 



