BOTIFERA 231 



the front end of the body and m the middle of which is the mouth, and 

 the special sense organs. The cilia are evenly distributed over the corona 

 in the most primitive rotifers; m others the corona is variously lobed and 

 the cilia are in groups and usually confined to the margin and the area 

 just within the margin. These marginal cilia in numerous common rotifers 

 whirl in opposite directions on the two sides of the corona and resemble 

 revolving wheels, giving the group its name. The special sense organs, 

 when present, consist of one to three eyes and one to four tentacles. The 

 trunk is in many rotifei^s encased in a shell called the lorica which is the 

 thickened cuticula; it is often provided with spines and other projections. 

 The foot forms the hinder portion of the body : it is usually retractile and 

 in most rotifers ends with one, two, or several toes. Glands are present 

 in it which secrete an adhesive substance by means of which the animal 

 can attach itself temporarily. 



The mouth opens into a large muscular pharynx called the mastax 

 in which are paired jaws or trophi, the working of which is a noticeable 

 feature in rotifers. In some rotifers (Stephanops) the pharynx is thrust 

 out of the mouth and used as a proboscis to take in food. A narrow 

 oesophagus joins the phaiynx with the large stomach, which has a pair of 

 large gastric glands and is joined with the dorsal anus by the short intes- 

 tine. In some forms the intestine ends blindly, there being no anus. The 

 nervous system consists of a brain dorsal to the pharynx and nen^es ex- 

 tending from it; a suboesophageal ganglion is present in some forms. A 

 pair of kidney tubules which contain flame cells open into a contractile 

 bladder, the vacuole, which communicates with the hinder end of the intes- 

 tine. The sexes are distinct. The males are small and without digestive 

 organs and usually much less numerous than the females ; in many species 

 they have not been found at all. The female has usually a single small 

 ovary and a large yolk gland which are joined with the cloaca by an ovi- 

 duct, the lower end of which acts as a uterus and retains the young, in a 

 large number of species, during development, so that they are born alive. 

 The females reproduce parthenogenetically : at certain times, however, 

 males are bom and the fertilized eggs then produced are called "winter^' 

 or resting eggs and can resist cold and drought. Budding and fission do 

 not occur. 



Habits and Distribution. — 'The majority of rotifers are solitary, free- 

 living animals, although a few species are sessile, living in tubes com- 

 posed of their own secretions or of foreign matters, and a few are colonial. 

 They are typically fresh-water animals and are everywhere abundant, but 

 a few species are marine. They are also usually rather rigidly confined 

 to certain environments, some living among plants and some being pelagic. 

 Most of the common species are cosmopolitan in their distribution. Some 



