268 



NEMERTEA. 



organs. The lateral nerve-cords are thickenings of this sheath, and 

 there is a median dorsal thickening constituting the proboscis nerve. 



In Heteronemertini the central organs are impregnated with 

 haemoglobin. As sense organs there are the lateral organs above 

 mentioned, the tactile hairs of the ectoderm cells, cephalic eyes 

 more or less numerous, and sometimes provided with refractive 



bodies. Exceptionally, as in 

 Oerstedia pallida, two oto- 

 lithic vesicles are found on the 

 brain. 



The vascular system consists 

 of three longitudinal vessels 

 with contractile Avails placed 

 just outside the intestinal wall ; 

 one of them is straight and 

 dorso- median, and the other 

 two are sinuous and lateral. 

 They communicate in front and 

 behind ; and anteriorly the 

 lateral vessels often dissolve 

 themselves in a network of 

 vascular spaces on the oeso- 

 phagus. The longitudinal 

 vessels give off lateral branches, 

 which no doubt open into a 

 system of lacunae in the tissues. 

 The blood is usually colourless, 



FIG. 222. Lateral organ of Drepanophoms 

 'i fin us (after Burger, from Perrier). c ecto- 

 derm ; b basement membrane ; v canal of the 

 organ ; dy dorsal lobe of cerebral ganglion ; 

 n nerve connecting dg to its posterior lobe ; 

 c point of division of the canal into a glan- 

 dular tube <?c, which projects back behind 

 the brain and into a canal cs which passes 

 into the dilatation s in the posterior lobe ; 

 ecs epithelium of the sac ; jii pigment. 



but in some species it is red. 



In Ampliiporus splendens and 

 Borlasia splendida, the red colour (haemoglobin) is contained in 

 the blood corpuscles. 



The excretory organs consist of two lateral tubes, which lie close 

 to the lateral vessel, and open externally by one, or rarely by several 

 openings on each side. They are confined to the anterior region 

 of the body, usually not extending further back than the oesophagus 

 (fore-gut). These tubes have a glandular ciliated lining, and in some 

 forms, possibly in all, give off branches which themselves branch 

 and finally end in small swellings, containing a long flame-shaped 

 cilium. These flame-cell ends are said to differ from the corre- 

 sponding structure in Turbellaria by consisting of many cells. As 

 already pointed out, this distinction is probably unimportant, 



