334 ZOOLOGY sect. 



Frequently, but not always, the twd tubes open posteriorly into 

 a contractile vesicle or bladder which discharges into the 

 cloaca. 



Nervous System and Sense Organs. — The nervous system 

 usually consists of a single ganglion (Fig. 267, Ir) towards the 

 dorsal aspect of the anterior part of the body, and representing the 

 brain or supra-cesophageal ganglion of the higher Worms : it sends 

 nerves to the muscles, trochal disc, and tactile organs, In some 

 cases a smaller ventral or infra-oesophageal ganglion is present 

 as well, connected with the first by a pan- of slender cesophageal 

 connectives. Connected with the dorsal ganglion are a pair 

 of lateral longitudinal nerves which run backwards to the tail, 

 giving off branches in their course. One or more eyes (e) are 

 usually present in close relation with the brain, and are sometimes 

 mere spots of pigment, but may be provided with a refractive 

 body or lens. The only other organs of sense are the tactile rods 

 {d./., l.f.), of which there is usually one on the dorsal surface near 

 the anterior end of the body, and frequently two others, 

 one on each side of the trunk. They are more or less rod-like 

 structures, tipped with delicate sensory hairs and receiving nerves 

 from the brain. 



Reproduction and Development. — In most cases the female 

 reproductive organs have the same general character as in Brachi- 

 onus, i.e. the gonad is unpaired (Fig. 264), consists of germarium and 

 vitellarium, and is provided with an oviduct (Fig. 267). But in 

 some of the Bdelloida, such as Philodina, there are two ovaries, not 

 divisible into germ-gland and yolk-gland, and the oviduct is absent. 

 The males are smaller than the females and degenerate in structure, 

 the enteric canal being atrophied (Fig. 266, A). There is a large 

 testis {t) with a duct opening at the end of a protrusible penis {p), 

 which is dorsal in all but Asplanchna, in which it, as well as the 

 cloacal opening of the female, appear to be ventral. Apparently 

 hypodermic impregnatio7i sometimes takes place, i.e. the body-wall 

 of the female may be perforated at any place for the entrance 

 of the sperms. 



Three kinds of eggs are produced : large and small sum^ner eggs, 

 which always develop parthenogenetically, the larger giving rise to 

 females, the smaller to males ; and thick-shelled ivinter eggs, which 

 probably require impregnation, and remain in an inert condition all 

 through the winter, finally developing in the spring. Most Rotifers 

 are oviparous, but some {Philodina, &c.) bring forth living young, 

 which are born by breaking through the body-wall or through the 

 cloaca, thus causing the death of the parent. 



Segmentation is total and irregular, the oosperm dividing into 

 megameres and micromeres. An epibolic gastrula is formed, the 

 blastopore closes, and invaginations of ectoderm give rise to the 

 stomodseum and proctodeum. The tail is formed as a prolongation 



