PHYLUM PLATYHELMINTIIES 



293 



amis. 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 Protonemcrtini and 



long.iie 



Fio. 234.— Diagrammatic transverse section of a ffemertean (Carinella). a,h,c. layers of 

 body-wall ; c. t connective tissue between body-wall and cntcron ; L he. lateral blood-vessels ; 

 loiifi. ne. longitudinal nerves ; p. proboscis ; p. s. proboscis-sheath, (After Hubrecht.) 



the Hetcronemertini is a nerve-plexus extending all over the body. 

 In the Mctancmcrtini, 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, 



latbly 



medMv 



Fig. 235.— Anterior portion of a Nemertean (Dropanophorus), showing the blood-va?cular 

 and excretory systems, lal. hi. v. lateral blood-vessels ; med. bl. v. median blood-vessels ; 

 Hfp/i. nephridial (excretory) tubes. (After Oudemans.) 



varies in the different groups. In the Protonemertini (Fig. 234.) 

 they occupy the most primitive position, being quite superficially 

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

 deeper : in the Metanemertini they lie in the parenchyma within 



