io6 



NEMERTINEA 



CHAF. 



nerve branches except the anterior ones to the head. In the 

 third group of Nemertines the lateral nerve-cords lie within the 

 muscular layers of the body-wall, and in this case paired nerve 



branches are given off at 



p-.v 



d.n 



d-,v 



V.C'-T- 



definite intervals throughout 

 the whole length of the body. 

 These branches divide up 

 among the organs to which 

 they pass, and no nerve 

 plexus is present. 



The lateral cords vary in 

 position in different cases. 

 Sometimes they lie laterally, 

 at others the cords tend to 

 approximate to one another 

 in the median dorsal or in 

 the median ventral line, 

 though in every case they 

 remain distinctly sepa- 

 \-l. v rated. 



Sense Organs. Sense 

 organs are usually present in 

 the form of eyes arranged at 



Fig. 55. Diagram to show the relations of 

 the nervous system, circulatory system, 

 and proboscis sheath in the anterior end of ] ie gxdes of the head (Fiff 

 the body in the Hoplonemertea, modified . i * 



from M'Intosh. a.n, Nerves to anterior 51, e), Sometimes as a Single 

 part of body and eyes ; d.c, dorsal commis- 

 sure ; d.n, median dorsal nerve ; d.v, dorsal 

 vascular trunk ; l.v, lateral vascular trunk ; 



nc, lateral nerve-cord; n.g.d dorsal lobe T j structure 

 of nerve ganglion ; n.g.v, ventral lobe ot 

 nerve ganglion ; p.p, proboscis pore ; p.s, 

 proboscis sheath ; v.c, ventral commissure ; 



vascular ring or collar. 



pair and sometimes in one 

 or more groups on each side, 

 the eyes 

 varies from a simple pigment 

 spot to an organ which re- 

 ceives a special nerve -supply 



from the brain, and possesses a refracting body answering to a 



lens, and behind this a pigment layer and a layer of rods. Some 



forms are devoid of all traces of eyes. 



A pair of simple auditory capsules has been found in some of 



the Hoplonemertea, where they occur as small vesicles on the 



brain. 



The whole surface of the body appears to be remarkably 



sensitive. In a few forms small tufts of tactile hairs are said 



to be present in the region of the head, while in others there 



