NEMERTEA. 



177 



approximations tend towards the two positions most characteristic of the 

 nervous systems of Annelids and Arthropods on the one hand, and of 

 Vertebrates on the other. 



Lateral 0?-gans. 



On each side of the head there is a ciHated pit communi- 

 cating with the exterior through an open sht or groove, and 

 communicating internally either with the brain itself, or with 

 adjacent and associated nervous tissue. In those cases in 

 which the development has been studied these so-called 



d. v.m. 



Fig. 57. — Transverse section of the Nemertean Drepanophonis 

 latus. (After BiJRGER.) 



(/.«., Dorsal nerve ; F.s.^ proboscis sheath ; Px., proboscis cavity ; 

 P.s.\ sac of proboscis cavity ; d.v.m., dorso-ventral muscles ; c.7n., 

 circular muscles ; /.;«., longitudinal muscles ; l.n., lateral nerve with 

 branches ; /"., parenchyma ; ,jr., gut ; /.z'., lateral blood vessel, beside 

 which excretory vessel; -£./-, excretory pore; d.z'.\ dorsal blood 

 vessel ; -£"/., epidermis. 



lateral organs arise from epiblastic insinkings and oesophageal 

 outgrowths. In the most primitive genus, Carinelia, they 

 are absent, except in one species. It has been suggested 

 that they conduce to the respiration of the brain, which is 

 rich in haemoglobin, and they have even been compared with 

 gill slits. In some forms the groove through which they 

 12 



