r8o MANUAL OF ZOOLOGY SECT. 



It is provided at its extremity in many forms with a pair of processes 

 which act like the blades of a pair of forceps in enabling the animal to 

 attach itself temporarily. In many forms in which the tail is well- 

 developed locomotion may be effected not only by swimming, owing to 

 the movements of the cilia of the trochal disc, but also by creeping or 

 looping 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. 



In some Rotifers the trunk is 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 com- 

 pletely 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 (ph.) 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 (br. ), situated towards the 

 oral end ; there are usually one or several very simple eyes (e. ) in close 

 relation to it, and one or several processes, the tactile rods (d.f. ), tipped 

 with non-motile cilia, are connected with the ganglion by means of 

 nerves. A pair of longitudinal excretory vessels (nph.) provided 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 pro- 

 duced : large and small summer eggs, which always develop without 

 fertilisation, 1 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, 



1 Such a mode of development without fertilisation is known as 

 parthenogenesis. 



