212 THE INVERTEBRATA 



free-swimming life. The dorsal surface of the rotifer is marked out 

 by the position of the cloacal aperture just in front of the foot; 

 on this surface immediately behind the velum is a sense organ, the 

 dorsal antenna^ and below it the hrain. There are also two lateral 

 antennae; all three are prominences bearing stiff sense hairs. Else- 

 where the body is covered by a thin, smooth, transparent cuticle 

 secreted by the ectoderm. 



The food, which consists of micro-organisms of various kinds, is 

 swept by means of the ciliary currents of the disc into the mouth and 

 then through the oesophagus into the muscular pharynx or mastax 

 which is provided with chitinous jaws, the trophi, which are in con- 

 stant movement and masticate the food as it passes through. This 

 first part of the alimentary canal is ectodermal and constitutes the 

 stomodaeum. Then follows the endodermal stomach, lined with ciliated 

 epithelium, in which digestion takes place. ^ Two gastric glands open 

 into it anteriorly. A narrow intestine leads into the cloaca, into which 

 the excretory system also opens. The latter consists of lateral ducts, 

 coiled at intervals, consisting of perforated cells placed end to end 

 into which flame cells (vibratile tags) open frequently but irregularly. 

 Anteriorly the ducts communicate by a transverse vessel just behind 

 the disc and posteriorly they open into a pulsating vesicle which 

 expels its contents into the cloaca. It has been calculated that in 

 some species this bladder expels a bulk of fluid equal to that of the 

 animal about every ten minutes. 



The single ovary is a bulky organ : it is divided into a small germar- 

 ium (the ovary proper) and a much larger vitellarium or yolk gland 

 which occupies much of the space between the stomach and the body 

 wall. The ovary is continued into a duct which opens into the cloaca. 



The female is still the only individual known in many kinds of 

 rotifers. It was not until 1848 that a male rotifer of any kind was 

 described. In only a few species is the male equal in size and organi- 

 zation to the female. In all the rest there is a more or less pronounced 

 sexual dimorphism. In Hydatina (Fig. 164 B) the male has no 

 alimentary canal, but the ciliated disc, musculature and excretory 

 system are well developed. Usually the male is not only smaller but 

 its ciliated disc and the alimentary canal are very much reduced and 

 the excretory system may be absent. The chief organ is the large 

 testis, usually filled with ripe spermatozoa, which opens by a median 

 dorsal penis in many cases. Where the penis is absent the tapering 

 hinder end may be inserted in the cloaca of the female. Finally, it may 

 be mentioned that in one large family, the Philodinidae, which in- 

 cludes the genus Rotifer, no male has ever been found. 



^ Digestion is usually extracellular, but in Ascopus and other rotifers it is 

 intracellular. 



