250 THE VITAL FUNCTIONS. 



from the intestinal canal, occurs in the Planarice, 

 which are a tribe of fiat vermiform animals, in 

 many respects allied to the more developed 

 Entozoa, and appearing placed as an interme- 

 diate link between them and the Annelida. In 

 many species, such as the Planaria nigra^ fusca, 

 and tremellaris, (Muller), Duges observed two 

 longitudinal trunks (Fig. 346*) running along 

 ^Q^ the sides of the under surface of 



the animal, and joining together, 

 both at their fore and hind ex- 

 tremities, so as to form a con- 

 tinuous channel of an oval form.t 

 A great number of smaller vessels 

 branch off from these main trunks 

 in every direction, and ramify ex- 

 tensively ; often uniting with those 

 from the opposite side, and esta- 

 blishing the freest communications 

 between them. 

 In the Annelida which have a more length- 

 ened and cylindric form, the principal vessels 

 have a longitudinal course ; but are difterently 

 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 



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

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



