288 



ZOOLOGY 



SECT. 



least, by the taking in and giving out of water through the mouth 

 by the oesophagus. 



The nervous system is in some respects more highly developed 

 than in the Turbellaria. The brain (Figs. 233 and 236 br., and Figs. 

 235, cer. g., and 240, n.g.d., n.g.v.) is composed of two pairs of 

 ganglia, dorsal and ventral, the ganglia of each pair being connected 

 together by commissures, the dorsal situated above, the ventral 

 below, the anterior part of the proboscis and proboscis sheath, and 

 both being above the mouth and oesophagus. From the brain 

 pass backwards a pair of thick longitudinal nerve-cords which 

 run .throughout the length of the body. Usually these are 



p. 



^-'- s.l 



d.b 





FIG. 239. Diagrammatic transverse section of a KTemertean (Heteronemertini) through 

 the middle region of the body. b.m. basement membrane ; c.m. circular muscle layer ; 

 d.b. dorsal blood-vessel ; ep. epidermis ; ff- gonads ; int. intestine ; l.b. lateral blood 

 vessel ; l.m. longitudinal mu,scle v layer ; n.c. lateral nerve cord ; n.L nerve plexus ; p. 

 proboscis ; p. s. proboscis aJj^th ; s.t. subcutaneous layer. (After Sheldon.) 



lateral in position^ sometimes approximated dorsally, sometimes 

 ventrally. The lateral nerve-cords generally meet posteriorly in a 

 commissure usually situated above, but in one genus below, the 

 anus. A third median dorsal nerve of smaller size than the 

 lateral cords extends backwards from the dorsal commissure of the 

 brain. Associated with the nerve-cords in the Protonemertini and 

 the Heteronemertini is a nerve-plexus (Fig. 239, n.L) extending all 

 over the body. In the Metanemertini y instead of a nerve-plexus, 

 there is a series of slender transverse connectives running across at 

 short intervals between the lateral nerve-cords, and from each cord 

 are given off numerous branches arranged with some regularity. 



The position of the brain and lateral nerve-cords and the 

 nerve-plexus, or the system of commissures and nerve-branches, 

 varies in the different groups. In the Protonemertini . they 

 occupy the most primitive position, being quite superficially 



