vii PHYLUM TROCHELMINTHES 321 



dimorphism. The preceding description applies to the female, which 

 is the form most commonly met with. In addition to the organs 

 already mentioned, it has a germarium (ov., ovy.), connected with a 

 large vitellarium (vt.) and opening by an oviduct into the cloaca. 



The male (Fig. 271, A) is a very minute creature, not more than 

 one-fourth the size of the female, and is strangely degenerate in 

 structure. The enteric canal is absent, the trochal disc simple in 

 structure, the nervous system and nephridial tubes greatly reduced, 

 and the greater part of the body occupied by a large testis (ts.) 

 which opens by a duct at the extremity of a protrusible, dorsally 

 placed penis (p.). 



After extrusion the eggs are attached to the base of the tail 

 of the female (B, ov'.), where they undergo development : they are 

 of two sizes, the larger giving rise to females, the smaller to males. 

 Probably both kinds develop parthenogenetically, but in the 

 autumn thick-shelled winter eggs are produced which appear to 

 require fertilisation. These remain quiescent during the winter, 

 and in the spring develop into females. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Rotifera are Trochelminthes of microscopic size. The 

 anterior end is modified into a retractile trochal disc, with variously 

 arranged cilia ; the posterior end usually forms a mobile and 

 often telescopically jointed tail. The mouth is anterior and more 

 or less ventral in position, the pharynx contains a chitinous 

 masticatory apparatus, and the anus is placed dorsally at the 

 junction of the trunk with the tail. There is a spacious body-cavity 

 devoid of epithelial lining. The excretory organs are a pair of 

 nephridial tubes provided with flame-cells. The central nervous 

 system consists of a single dorsal ganglion, with, in a few cases, a 

 smaller ventral or sub-cesophageal ganglion. The sexes are separate, 

 and the males are, in nearly all cases, smaller than the females 

 and degenerate in structure. 



The class is divided into six orders as follows : 



ORDER 1. EHIZOTA. 



Rotifera which are fixed in the adult state by the truncated end 

 of the non-retractile tail. 



Including Floscularia, Stephanoceros, Melicerta, etc. 



ORDER 2. BDELLOIDA. 



Rotifera which both swim freely by means of the cilia of the 

 disc and creep after the manner of a Leech. The tail is telescopic 

 and forked distally. 



Including Rotifer, Philodina, etc. 



VOL. I. Y 



