vn PHYLUM TROCHELM1NTHES 327 



or the latter may be a distinct chamber of more or less globular 

 form. In the Rhizota the intestine turns forwards so as to allow of 

 the anus being brought over the edge of the tube in defsecation 

 (Fig. 274, 4, a). In Asplanchna (6) the stomach ends blindly, the 

 intestine, cloaca, and anus being absent. 



The excretory system is very uniform in structure. It con- 

 sists of a pair of more or less coiled nephridial tubes, placed 

 longitudinally and giving off lateral branchlets which end in 

 flame-cells. The outer surface of each flame-cell usually bears 

 one or sometimes two flagella, which lie free in the body-cavity. 

 Frequently, but not always, the two 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. 272, br.) towards the 

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

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

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

 cases a smaller ventral or infra-oasophageal ganglion is present 

 as well, connected with the first by a pair 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.f., 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. 269), consists of germarium and 

 vitellarium, and is provided with an oviduct (Fig. 272). 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. 271, 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, appears to be ventral. Apparently 

 hypodermic impregnation 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 summer eggs, 

 which always develop parthenogenetically, the larger giving rise to 

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



