28 OLIGOCHAETA 



dorsal vessel does not extend throughout the body ; 'it is connected with the general 

 coelomic cavity by a ventrally placed slit situated anteriorly, just where it narrows 

 to pass through the intersegmental septum. 



The dorsal vessel is not the only vessel which is enclosed in a special perihaemal 

 space ; in the Eudrilid Libyodrilus, the two sub-intestinal vessels are enclosed in 

 a space of the same kind ; two mesenteries arise from the ventral wall of the oesophagus, 

 which meet below and shut oif a space with a crescentic outline ; in this space run 

 the two vessels in question. For a part of their course the vessels are free in the 

 interior of the space, further back they are attached to the walls of the space, and 

 further back still they come to lie outside of them. The spaces in fact are each of 

 them confined to a segment, and do not pass continuously from segment to segment. 



In the two closely allied genera Hdiodrilus and Hyperiodrilus, the supra-intestinal 

 vessel is in the same way enclosed in a coelomic space, distinct from the general coelomic 

 cavity. As in the case of the dorsal vessel of Beinodrilus, the walls of the perihaemal 

 space are connected here and there with the walls of the contained blood-vessel by 

 delicate strands of fibrous tissue. The interspaces of these are filled with corpuscles. 

 It seems possible that the function of these perihaemal spaces is concerned with the 

 formation of the coelomic corpuscles ; they were always found to be filled with 

 corpuscles, and in more than one instance the corpuscles could be observed in the 

 act of being budded off from the walls of the space. 



§ 2. Other subdivisions of Coelom. A subdivision of the coelom only paralleled 

 in the Polychaeta occurs in the genus Libyodrilus and in the aquatic Branchiura. 

 In the former worm the two pairs of seta of each side of the body arise from the 

 floor of a chamber which is cut oif from the general coelomic cavity. There are thus 

 a pair of chambers along the body like the parapodial chambers in certain Polj'chaeta. 

 The membrane which forms the wall of these chambers is thin and presents no 

 appearance of structure except externally, where it is covered by nuclei ; the nuclei 

 are on both sides ; the membrane is continuous with the parietal peritoneum ; the 

 band of muscles uniting the two pairs of seta lies well below the membrane, which 

 in section is seen to be somewhat, though not greatly, arched. Something of a similar 

 kind occurs in the Tubificid Branchiura. Here the body at least in the posterior 

 region is hourglass-shaped in transverse section ; from the ' waist ' of the hourglass 

 a septum runs across the body-cavity transversely, dividing it into an upper chamber 

 which contains the gut, and a lower chamber in which lie the nervous system and 

 both dorsal and ventral blood-trunks. 



In addition to the coelomic tubes which have been described as surrounding some 

 of the blood-vessels in certain Oligochaeta there is in Allolohophora a ventral tube 



