VII 



PHYLUM TROCHELM1NTHES 



303 



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. 



c.yl 



FIG. 248. Diagram of a Rotifer, n. aim.* ; ///. brain ; r 1 . pras-oral ; c 2 . post-oral circlet of cilia ; 

 '. ///. cement gland ; cl. cloaca ; cw. cuticle ; d.ep. deric epithelium ; <l. f. dorsal feeler ; e. eye ; 

 H. r. flame-cells ; int. intestine ; ui. muscles ; mtJt. mouth ; ,iph. iiephridial tube; ov. ovum ; 

 oi-il.. oviduct; ory. ovary ; jik. pharynx ; .sf. stomach ; vt. vitellarium. 



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 coelome. 

 The excretory organs are a pair of nephridial tubes provided with 

 flame-cells. The , nervous system consists of a single dorsal 

 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 five orders as follows : 



ORDER 1. RHIZOTA 



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

 of the non-retractile tail. 



Including Floscularia, Stcphanoceros, 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, Pliilodina, etc. 



