CIRCULATION IN THE ANNELIDA. 181 



tinal canal, occurs in the Pfanarise, which are a tribe of flat 

 vermiform animals, in many respects allied to the more 

 developed Entozoa, and appearing placed as an intermediate 

 346* link between them and the Annelida. In 

 many species such as the Planaria nigra y 

 fusca, and tremellaris, (Muller,) Duges ob- 

 served two longitudinal trunks (Fig. 346*) 

 running along the sides of the under surface of 

 the animal, and joining together, both at their 

 fore and hind extremities, so as to form a. 

 continuous channel of an oval form.f A great 

 number of smaller vessels branch off from 

 these main trunks in every direction, and 

 ramify extensively, often unitjng with those 

 from the opposite side, and establishing 

 the freest communications between them. 

 In the Annelida which have a more lengthened and cy- 

 lindric form, the principal vessels have a longitudinal course, 

 but are differently disposed in different species. There is, 

 in all, a vascular trunk, extending along a middle line, the 

 whole length of the back, and especially designated as the 

 dorsal vessel: in general, there is also a corresponding trunk, 

 occupying the middle line of the lower, or abdominal side 

 of the body, and termed the abdominal vessel. This latter 

 vessel is sometimes double; one being superficial, and ano- 

 ther lying deeper; the principal nervous cord, and chain of 

 ganglia being situated between them. Frequently, there 

 are found, in addition to these, vessels which run along the 

 sides of the body, and are therefore called the lateral ves- 

 sels. In every case there are, as we have seen in the Plana- 

 ria, numerous branches, and collateral communications be- 

 tween the lateral, the abdominal, and dorsal vessels; more 

 especially at the two extremities of the body, where the great 

 mass of blood, which has been flowing in one direction in 

 one set of vessels, is transferred into others, which convey 



f De Blainville has described a structure similar to this in a Planaria from 

 Brazil. Diet, des Sc. Nat. t. xli. 216. 



