TUBIFEX. 327 



lining. Its place in these lower forms is taken by a 

 delicate membrane, destitute, apparently, of any cellular 

 structure. Beddard described the same condition in other 

 aquatic worms, viz., Naiads and Enchytrgeidae. It is 

 only in the true earthworms that this membrane is 

 replaced by an epithelium consisting of large and 

 conspicuous cells. In the smaller vessels, such as the 

 perivisceral and intestinals, the wall consists of nothing 

 more than this membrane covered externally by a single 

 layer of flattened, peritoneal cells. Such branches must 

 necessarily be non-contractile. The walls of the main 

 blood-vessels show a rather more complicated structure, 

 but still remain very thin and delicate. The membrane, 

 mentioned above, is supported externally by a few muscle 

 fibres. The circular muscles are very few, and are 

 situated just outside the membrane. The longitudinal 

 fibres are better developed, and lie just underneath the 

 peritoneal epithelium, which forms the outermost covering 

 of the wall. 



Owing to the rapid movements of the worm, the dorsal 

 and ventral vessels are constantly thrown into a number 

 of zig-zags or S-shaped portions, which are rendered much 

 more apparent by the addition of chloroform or ether to 

 the water in which the worm is placed, when the move- 

 ments of the worm become very violent. 



The main trunks which have a longitudinal direction 

 are the dorsal, supra-intestinal, and ventral vessels. 



1. The Dorsal Vessel extends through the entire 

 length of the body from the anal segment to the 

 prostomium (PL I, fig. 1, d.v.). It lies dorsal to the 

 alimentary canal for the greater part of its length, but 

 in the region of the reproductive organs it changes its 

 position and comes to lie nearer the ventral vessel. 

 Further back it reassumes its original position. 



