230 



ROTIFERA 



CHAP. 



strong cilia, separated by a . finely ciliated groove, like those oi 

 the ciliary wreath of a Eotifer. The sides are produced oi 

 either side into lappets, which we do not take into account, 

 cup-shaped depression at the apical pole is lined by sense-cells, 

 bearing long cilia which are probably sensory. A ring of nerve- 

 cells passes within the ciliated rim of the hemisphere, and th( 

 stomach is a blind sac. If we compare this organism with a 

 Eotifer, we find that the wreath corresponds in both, the funnel 



CUt^rO. 



118. Diagram explaining the 

 possible relations of Rotifers. 

 Pilidium; B, hypothetical Rotifer 

 modified, from Asplanchnopus ; 

 a Ploimal Rotifer ; D, Trochosplu 

 aequatorialis (modified from Sempe 

 the extension of the ovary into 

 the posterior ventral quadrant 

 being omitted) ; E, Mollusc larva 

 (Veliger) ; F, Trochophore larva of 

 Annelid. a, Anus ; ap, apical 

 organ ; at, median antenna (near 

 which, in B, is a black spot, the 

 brain) ; bl, bladder (receiving the 

 ramified kidney in B, C, and D) ; br, 

 brain ; /, foot ; fg, cement-glands, 

 replacing apical organ ; g, ovary ; 

 k, kidney ; m, mouth ; n, supra- 

 oesophageal ganglion ; nap, nerve 

 of apical organ ; nr, nerve-ring in 

 section ; pot, praeoral portion of 

 trochus ; s.g, shell-gland ; s.n, sub- 

 oesophageal ganglion ; t, trochus or 

 ciliary wreath ; tt, posterior ciliated 

 ring ; r, velum, or expanded prae- 

 oral part of trochus. 



of the disc in such forms as Flosculariidae and Microcodon lead- 

 ing to the mouth of Pilidium, while the gut is blind in Asplanch- 

 nidae and in some of the highly developed Seisonidae. The 

 circular nerve-ring of Pilidium is in many Eotifers only repre- 

 sented by its anterior part, the brain ; though in Bdelloids a sub- 

 oesophageal ganglion completes the ring. This leaves a difficulty 

 with regard to the apical sense organ ; but it is easy to understand 

 that an organ of sensation should become an organ of fixation. In 

 this case the foot with its glands would correspond to the sense 

 organ of the Trochophore larva ; and it retains its primitive ciliated 

 character in the larvae and males of many Eotifera, and the adult 

 female of Pterodina and Callidina tetraodon. Embryology tells 



