240 THE INVERTEBRATA 



of revolving wheels, the reason for the scientific name of the group. 

 In Hydatina the cingulum forms a complete ring and the trochus is 

 reduced to a double transverse row of cilia; in the groove between 

 them is situated a number of papillae on which are stiff cilia. (In 

 Copeus and other creeping forms there are no trochus and cingulum 

 but cilia cover the trochal region and part of the ventral surface. This 

 is said to be a primitive arrangement.) The posterior end is called the 

 foot and it terminates in a pincer-shaped appendage, on which open 

 glands with a sticky secretion. By means of this apparatus the rotifer 

 can anchor itself in the intervals of its free-swimming life. The dorsal 

 surface of the rotifer is marked out by the position of the cloaca! 

 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 brain. 

 There are also two lateral antennae ; all three are prominences bearing 

 stiff sense hairs. Elsewhere 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 trophic which are in con- 

 stant movement and, in Hydatina^ 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 

 two 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 gerinar- 

 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. 170 B) the male has no 



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

 intracellular. 



