PHYLUM PLATYHELMINTHES 271 



commissures, dorsal and ventral, between which pass the proboscis 

 and its sheath. From the brain pass backwards a pair of thick 

 nerves which run throughout the length of the* body; usually. 



c.t 



c.t 



lang.n 



long.ne 



l.Tyv 



FIG. 219. Diagrammatic transverse section of a Nemertean (Carinella). a,b,c. layers of 

 body-wall ; c. t. connective tissue between body-wall and enteron ; 1. bv. lateral blood-vessels ; 

 long. ne. longitudinal nerve ; p. proboscis ; p. s. proboscis sheath. (After Hubrecht.) 



these are lateral in position, sometimes approximated dorsally, 

 sometimes ventrally. In the Nemerteans devoid of stylet there is 

 a nerve-plexus between the muscular layers. In the sty let -bearing 

 forms such a plexus is absent, but metamerically arranged branches 

 are given off by the nerve-cords ; these divide into smaller nerves- 



mph 



tetblv 



mzd.hlv 



FIG. 220. Anterior portion of a Nemertean (Drepanophorus), showing the blood-vascular 

 and excretory systems. lot. bl. v. lateral blood-vessels ; rued. bi. v. median blood-vessels ; 

 neph. nephridial (excretory) tubes. (After Oudemans.) 



for the supply of the various organs. Sometimes the lateral cords 

 unite behind above the anus. 



A remarkable apparatus connected with the nervous system is 

 the system of lateral organs. These consist of a pair of ciliated 

 tubes (Fig. 215, cil. gr.) opening externally at the sides of the head, 



