240 OLIGOCHAETA 



vessel in earthworms. It lies in close contact with the oesophagus, beneath the 

 peritoneum covering this organ. A supraintestinal vessel is present in the genera 

 Tubifex, Psammorydes, Spirosperma, Loplwchaeta, Limnodrilus, Bothrioneuron, and 

 Branchiura; it is absent in Ilyodrilus; with regard to the remaining genera there 

 is no information. The supraintestinal vessel, as in many, if not in all, earthworms, is 

 limited to the oesophageal region ; it dies away gradually in front and behind. 



The ventral vessel is present in all Tubificidae, and is quite free from the wall of 

 the alimentary tract. It is (apparently always) non-contractile. Most Tubificidae (the 

 genera Bothrioneu,ron, Lophochaeta, Tubifex, Psammoryctes, Spirosperma, and Limno- 

 drilus) possess also a subintestinal vessel, corresponding, on the ventral side of the 

 oesophagus, to the supraintestinal on the dorsal side. In Bothrioneuron and 

 Lophochaeta, at any rate, this vessel communicates in front with the ventral 

 vessel. The subintestinal vessel may, perhaps, be the equivalent of the intestino- 

 tegumentary trunks of earthworms. 



The dorsal and ventral vessels communicate, in nearly every segment of the 

 body, by perivisceral arches. In the anterior segments of the body these are, 

 as a rule, larger, and some of them are specially dilated, and constitute the so-called 

 hearts. This, again, is a character met with in earthworms, as is also the contractility 

 of some of the anterior perivisceral arches. In Telmatodrilus and Branchiura five 

 or six pairs of these arches are contractile ; these, in Telmatodrilus, gradually increase 

 in calibre in successive segments from before backwards ; in Branchiura those of 

 ix and x are specially dilated. In the latter genus segment viii contains, in addition 

 to the perivisceral loops connecting the dorsal and ventral vessels, a vascular arch, 

 uniting the supraintestinal with the ventral vessel. In other Tubificidae there are one 

 (Tubifex, Lophochaeta) or two (Limnodrilus, Bothrioneuron) contractile trunks, which 

 pass round the oesophagus and unite the supraintestinal with the ventral vessel. The 

 occurrence of these vessels is highly interesting. They evidently correspond to the 

 'intestinal hearts' of earthworms (Pontodrilus, Perichaeta, &c.) which put into 

 communication the supraintestinal and ventral vessels (being sometimes, moreover, 

 also connected with the dorsa;l vessel) ; and it will be noted that, as in earthworms, 

 they are the last of the series, and are specially dilated. 



In all Tubificidae the alimentary tract is surrounded by a network of blood- 

 capillaries, which are derived, anteriorly from the supraintestinal, and posteriorly 

 from the dorsal, vessel. In Ilyodrilus, as in the Naidomorpha, the intestinal 

 network communicates with the ventral vessel by a pair of trunks in every segment. 

 In other Tubificidae there seems to be no connexion between the intestinal network 

 and the ventral vessel. 



