CLASS ROTIFERA. 193 



of the body-wall, supplied with muscles for its retraction, and 

 to which nerve-fibres pass from the supraoesophageal ganglion. 

 In a few forms, such as Melicerta, the calcar is double. 



No blood vascular system exists, but a well-developed 

 excretory apparatus (JV), resembling that of the Turbellaria, 

 is present. It consists of two longitudinal tubes, one on each 

 side of the body, from which arise a varying number of finer 

 lateral branches, each of which terminates in a funnel closed 

 by a flame-cell, as in the Turbellaria. Anteriorly the two 

 tubes may be united by a transverse connecting tube, and 

 posteriorly they may unite together to form a contractile 

 bladder which opens into the cloaca, or in some cases m&y 

 open directly to the exterior. 



The female reproductive apparatus consists of a relatively 

 large ovary (Ov) which in some cases at least consists of a 

 vitellariuni portion and an ovary proper, the whole being 

 surrounded by a thin membrane a backward prolongation of 

 which forms an oviduct opening into the cloaca. 



The preceding description of the structure of a Rotifer is 

 that of such a form as is most frequently met with. It was 

 for a long time believed that these were hermaphrodite, but 

 no trace of a testis could be found. It was later found, how- 

 ever, that they were all females, and the males of several 

 species have been discovered, differing decidedly in size and 

 structure from the females, and besides being usually rather 

 rare in their occurrence. They are considerably smaller than 

 the female, and possess like it eyes, nerve-ganglion, muscles, 

 and excretory system ; but the ciliated band of the trochal 

 disk is single, and the digestive tract, with the exception of the 

 cloaca, is reduced to a solid band of tissue. The single testis 

 occupies the greater portion of the body-cavity, and the short 

 vas deferens opens into the cloaca, passing through an avertible 

 intrornitteut organ. This marked difference of form of the 

 male and female individuals of the same species constitutes 

 a phenomenon known as sexual dimorphism. An explanation 

 of the usual numerical preponderance of the females over the 

 males is to be found in the fact that under favorable condi- 

 tions the females produce ova capable of developing parthe- 

 nogeuetically, and giving rise in all cases to females. A series 



