Class 4. Nemertinea. 



155 



already mentioned^ there are auditory vesicles in a few forms. 

 In contrast to other Flatworms, the Nemertines possess a vascular 

 system, usually consisting of three principal longitudinal trunks, 

 two lateral, and one dorsal, which 



come into relation with one p /u- 



another anteriorly and pos- 

 teriorly, and are besides con- 

 nected by transverse vessels. 

 The blood flows from behind 

 forwards in the dorsal vessel, 

 from before backwards in the 

 lateral vessels ; it is colourless 

 or red. The Nemertines are 



almost always or separate Fig. lie. Diagrammatic transverse section 



sexes; the sexual organs are °^ ^ Nemertine. d Dorsal vessel, g gonad, 



, . , . , h skin, i gut, I lateral vesEel, Jm longitudinal 



much Simpler m structure than muscle, « lateral nerve cord, p proboscis, ps 



those of other Platyhelminths. proboscis sheath, r circular muscle. 



Many ovaries or testes are 



present, opening by one duct on the side of the body. There are 



no accessory and no copulatory organs. 



Some Nemertines undergo a metamorphosis, which is 

 aberrant, in that the principal part of the larval body atrophies. 

 The larva, which is sometimes of a peculiar form (Fig. 117) is free- 

 swimming. 



Kg. 117. Three larval stages of a, Nemertine; the oldest larva (C) is the 

 so-oaUed pilidium. c flageUum, g gastrula mouth, e and e' invagination of the sMn, from 



which a great part of the final Nemertine body arises ; m stomach, oe oesophagus. After 



Metsohnikoff. 



