io6 



NEMERTINEA 



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 

 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- 

 rated. 



Sense Organs. Sense 



organs are usually present in 

 FIG. 55. Diagram to show the relations of ,, ., 



the nervous system, circulatory system, the form of eyes arranged at 



and proboscis sheath in the anterior end of fc ne sides of the head (Fit'. 



the body in the lloplone.mertea, modified 



from M'Intosh. a.n, Nerves to anterior 51, 6), Sometimes as a Single 



part of 'body and eyes ; d.c, dorsal 1 comrnis- pair and sometimes in One 



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



vascular trunk ; Lv, lateral vascular trunk ; or more groups Oil each side. 



The structure of th< 



. 



nerve ganglion ; p.p, proboscis pore ; p.s, varies from a simple pigment 



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 



