PHYLUM PLATYHELMINTHES 



289 



-\ v.s 



situated at the bases of the epidermal cells. In the rest they are 

 deeper : in the Metanemertini they lie in the parenchyma within 

 the muscular layers. The median cord is always, except in the 

 Heteronemertines, superficially placed. 



A remarkable apparatus connected with the nervous system 

 is that composed of a pair of peculiar structures known as the 

 cerebral organs. When most highly developed these consist 

 of a pair of ciliated tubes, opening externally in the region of the 

 brain and terminating internally in close relation to the dorsal 

 ganglion of the brain or a special ganglion distinct from the latter. 

 The external aperture may be 

 situated in a groove or furrow 

 (Fig. 235, til. gr.), vertical or 

 horizontal, of varying extent. 

 This apparatus may have a 

 respiratory function, more 

 especially for the oxygenation 

 of the substance of the brain, 

 but perhaps it has also a 

 sensory function. It has some 

 resemblance to the ciliated 

 pits developed in certain 

 Turbellaria. 



Eyes are present in the 

 majority of Nemerteans, and 

 in the more highly organised 

 species occur in considerable 

 numbers. Sometimes they 

 are of extremely simple struc- 

 ture ; in other cases they are 

 more highly developed, having 

 a spherical refractive body 

 with a cellular " vitreous 

 body," and a "retina" con- 

 sisting of a layer of rods 

 enclosed in a sheath of dark 

 pigment, each rod having a 

 separate nerve-branch connected with it. Statocysts containing 

 statoliths have been found in only a few of the Nemerteans. 



Reproductive System. Most species are dioecious. The 

 ovaries (Fig. 233, ov.) and testes are situated in the intervals between 

 the intestinal caeca. The ovary or testis is a sac the cells lining 

 which give rise to ova or spermatozoa ; when these are mature each 

 sac opens by means of a narrow duct leading to the dorsal, rarely 

 to the ventral, surface, 'on which it opens by a pore. In all 

 probability the cavities of these hollow gonads are all that 

 represent the ccelome of higher forms. 



VOL. i. u 



l.n- 



FIG. 240. Diagram of anterior end of a 

 Nemertean (Metanemertini). a. n. anterior 

 nerves ; d. c. dorsal commissure ; d. n. dorsal 

 median nerve ; d. v. dorsal vessel ; /. v. lateral 

 vessel ; n. c. lateral nerve-cord ; n. d. g. dorsal 

 ganglion ; n. g. v. ventral ganglion ; p. p. pro- 

 boscis pore ; p. a. proboscis sheath ; v. c. ventral 

 commissure ; v. s. vascular ring or collar. 

 (From Sheldon, modified from Mclntosh.) 



