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THE VITAL FUNCTIONS. 



346* 



blood are thus continually intermixed in the course 

 of the circulation.* This is well exemplified in the 

 PlanaricB, which are a tribe of flat vermiform ani- 

 mals, in many respects allied to the more developed 

 Entozoa, and appearing placed as an intermediate 

 link between them and the Annelida. In many 

 species, such as the Planaria nigra, fusca, and tre- 

 mellaris, (Muller), Duges observed 

 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 con- 

 tinuous channel of an oval form. 

 A great number of smaller vessels 

 branch oft' from these main trunks in 

 every direction, and ramify exten- 

 sively ; often uniting with those from 

 the opposite side, and establishing the 

 freest communications between them. 

 In the Annelida which have a more lengthened 

 and cylindric form, the principal vessels have a 

 longitudinal course ; but are differently disposed 

 in diffierent 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: sometimes there are two such 

 vessels, one on each side of the median line ; and 

 frequently these two vessels unite to form a single 

 trunk as they proceed towards the head. In ge- 

 neral there is also a corresponding trunk, occu- 



* For the clearest account of the circulation in the Annelida, see 

 a memoir by Milne Edwards, in the Annales des So. Nat. serie 2, x, 

 193. 



