LINEUS. 253 



and becoming a true vessel, runs for the rest of its extent 

 between the proboscis sheath and the intestine. In the 

 intestinal region the lateral and median dorsal vessels 

 communicate by a series of commissures (PI. IV., fig. 1, 

 cbv) passing round the diverticula, and which are of 

 lacunar nature, being devoid of the investing layer of 

 parenchymatous cells. As the lateral vessels do not com- 

 municate ventrally, the last commissure of the vascular 

 system lies dorsal to the alimentary canal. 



The blood is colourless, and contains some corpuscles. 

 In some Nemerteans it is red from the presence of 

 haemoglobin. As to its course, it has been stated to flow 

 backwards in the median dorsal and forwards in the 

 lateral vessels. It is possible, however, that, in the 

 absence of contractile fibres in the vascular system, there 

 is no definite circulation, but that the blood is inter- 

 mittently kept in motion by contraction of the muscles of 

 the body wall. To what extent the vascular fluid acts as 

 a respiratory medium is open to question. The vascular 

 network round the oesophagus of many Nemerteans and 

 the occasional presence of haemoglobin suggest that it may 

 have such a function, but against this must be set the fact 

 that the oesophageal lacunae are not present in some 

 groups, and also that the presence of haemoglobin is of rare 

 occurrence. A more likely view is that respiration is 

 mainly, if not entirely, carried out b}^ the integument of 

 the smaller forms ; and this may also be the case even in 

 the larger ones, since the body is usually capable of 

 extreme attenuation. 



EXCRETORY SYSTEM. 



The so-called excretory or nephridial system in Lineus 

 gesserensis consists of a number of small tubules lying in 

 close proximity to the lateral lacunae in the oesophageal 



