CIRCULATION IN THE ANNELIDA. 



181 



tinal canal, occurs in the Plnncmre^ which are a trihc of flat 

 vermiform animals^ in many respects allied to the more 

 developed Entozoa, and ap])earing placed as an intermediate 

 3-16* link between them and the Annelida. In 



many species such as the Vhinaria nigra^ 

 fitsca, and tremellaris, {Muller,) Duges ob- 

 served two longitudinal trunks (Fig. 346*) 

 running along tlie 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 extensivel}^, often uniting 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 alons: 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 bod}-, 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 Blainvillc has described a structure similar to this In a Planaria from 

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



