158 Introduction to Animal Morphology. 



g-angliform swelling. In many Dendrocoelans, the 

 feeble nervous system has no detectible ganglion cells, 

 and lies within the primitive vascular canals (water- 

 vascular system). Reddish or black photoscopic eyes 

 ■exist in many, either few on the neck or many mar- 

 ginal and clustered ; refracting bodies are added in 

 Vortex, Mesostomum, &c., and rod-like processes in 

 Geodesmus. In eyeless Rhabdocoela, and a few 

 Nemerteans, there is sometimes, close to the ganglion, 

 a single clear ear-vesicle ciliated within, containing 

 a spherical otolith. This rarely co-exists with ocelli 

 (CErstedtia pallida, Monocelis anguilla). 



Rod-like bodies lying on the ganglion in a few forms 

 may be touch-organs. In Nemerteans, there is on each side of 

 the front end of the body one or two ciliated grooves (Fig. 21,^), 

 which may be shallow, conical, or with margins capable of 

 closure, on whose floor is a ganglion clothed by ciliated pro- 

 toplasm. The roots of these ganglia arise from the side 

 (Nemertes), back (Cerebratulus), or front (Polia humilis) of 

 the pharyngeal nerve centre, or from the lateral nerve cords 

 (P. bembix). These are regarded as smell-, taste-, or touch- 

 organs, or as the mouths of excretory ducts (?) ( Van Benedeii). 

 Traces of similar grooves exist in Microstomidae, Polygordius, 

 &c. They are absent in Cephalothrix, &c. In Carinella 

 annulata, a frill exists representing the upper lip of the 

 cephalic fissure. At the ends of the grooves are two lateral 

 sacs, with a funnel-like ciliated duct. The sac wall has a 

 streak of finely granular ciliated cells. In Bipalium, the head 

 possesses rows of papillae and ciliated pits. 



Reproduction occurs rarely by transverse fission 

 (Microstomum), freely by artificial division, by conti- 

 nuous gemmation producing a tape-worm-like form 

 (Derostomum catenula), but commonly by ova. Most 

 are hermaphrodite, often self-impregnating, with a 

 common (most Rhabdocoela) or separate sex-openings 



