MI PHYLUM TROCHELMINTHES 327 



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. 266, 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 



a 



a, 



FIG. 267. Diagram of a Rotifer, a. anus ; 6r. brain ; c 1 . pre-oral, and c 2 . post-oral circlet of cilia ; 

 c. gl. cement gland ; cl. cloaca ; cu. cuticle ; d. ep. deric epithelium ; ll.f. dorsal feeler ; e. eye ; 

 ./!. r. flame-cells ; int. intestine ; m. muscles ; mth. mouth ; nph. nephridial tube ; ov. ovum ; 

 ovd. oviduct ; ovy. germarium. ; ph. pharynx ; st. stomach ; vt. vitellarium. 



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 ante- 

 rior 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 



