GENERAL ACCOUNT OF NEMERTEA. 



171 



systems of Annelids and Arthropods on the one hand, and of Verte- 

 brates on the other. 



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

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

 communicating internally either with the brain itself or with 

 adjacent nervous tissue. In those cases in which the 

 development has been studied, these so-called lateral organs 



d.n 



d. v. m. 



Fig. 76. — Transverse section of the Nemertean Drepanophorus latus. 

 — After Burger. 



d.n. y Dorsal or proboscis nerve; P.s., proboscis sheath; P.c, proboscis 

 cavity; P.s'. , sac of proboscis cavity; d.v.m., dorso-ventral muscles; 

 cm., circular muscles; /.;//., longitudinal muscles; /.«., lateral nerve 

 with branches; P., parenchyma; g., gut; Lv., lateral blood vessel, 

 beside which lies an excretory vessel ; E.p., excretory pore ; d.if. , dorsal 

 blood vessel ; Ep. , epidermis. 



arise from epiblastic insinkings and oesophageal outgrowths. 

 In the most primitive genus, Carinella, 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 open 

 to the exterior is rhythmically contractile. It has also been 

 suggested that they are sensory. Apart from these organs, 

 Nemertines are very sensitive, and in many this is associated 



