Annelidis. 325 



from being so mixed up with the dark pigment, they cannot be very 

 readily made out : this is more particularly true as regards the trunk 

 situated on the dorsal surface, the one on the ventral surface, from its 

 being intimately connected with the nervous cord, is very readily dis- 

 tinguished. The dorsal vessel, which it will be necessary first to de- 

 scribe, extends from the head to the tail, along the middle line of the 

 back, being very much obscured by pigment and cellular tissue. It 

 is about -^V of ^^ inch in diameter, and appears as a black line running 

 down the middle of the body. It receives the blood from the lateral 

 vessels by branches named dorso-lateral, and communicates freely 

 with the branches of the abdominal vessel ; about the posterior third 

 of the body it also receives a large branch, which comes off from the 

 intestines ; this branch runs parallel with the main trunk, w^hich then 

 proceeds to the head, where it receives numerous branches from the 

 oesophagus and parts around the mouth. In the whole of its course 

 it receives a pair of branches at each segment of the body, which cor- 

 respond in arrangement with the branches of the abdominal or ven- 

 tral vessels. 



The ventral vessel accompanies the nervous cord in nearly the 

 whole of its course. It commences at the head, where it receives nu- 

 merous branches from the sucking disk, and from the oesophagus and 

 oesophageal nervous ganglion or brain; it also receives branches on 

 each side of the body, at those parts where the nervous chord forms 

 a ganglion ; these branches accompany the nerves coming off from 

 the ganglia, and so close is the analogy in the arrangement between 

 the vessels and nerves, that many authors have confounded the two, 

 some describing it as a blood-vessel, and others as the nervous cord. 

 Dr. Rawlins Johnston* appears to have been the first to notice the 

 ventral vessel. He describes it in the medicinal leech as a pulsating 

 vessel, and as forming expansions in its course, where it assumes the 

 figure of a diamond. According to this view of the subject, these ex- 

 pansions must correspond with the nervous ganglia ; he mentions in 

 his description of the nervous system of the same animal, that the 

 nerve has diamond-shaped expansions, which correspond with those 

 of the blood-vessel ; or in other words, that this vessel forms a sheath 

 for the nervous cord, (fig. 2). This opinion has been followed by 

 Brandt and many other authors, all of whom state that the nervous 

 cord is being continually bathed in the venous blood. Some authors 

 make allusion to the nervous cord, and have overlooked the blood- 



* Treatise on the Medicinal Leccli, p. 115. 



