1 62 Elementary Zoology. 



develop muscles in the walls, which then contract, driving the 

 contained blood from place to place. In the earthworm the 

 dorsal vessel and the lateral hearts are thus contractile, and 

 play the part of the heart of higher types. The trunks of the 

 vascular system are usually arranged in a longitudinal direction, 

 the trunks being connected by transversely arranged vessels. 

 Such an arrangement is well seen in the earthworm. In this 

 animal it clearly shares the general segmentation of the body. 

 In the crayfish, which is also a segmented animal, but one in 

 which the segmentation of the internal organs is commencing 

 to be obscured, the metameric arrangement of the vascular 



Fig. 75. — Isolated capillary network formed by the junction of several hollowed-out cells. 

 (From Quain's " Anatomy.") 



f, a hollow cell the cavity of which does not yet communicate with the network ; p,p^ 

 pointed cell-processes, extending in different directions for union with neighbouring 

 capillaries. 



system is still obvious, but not so clear as in the earthworm. 

 There is no longer a metamerism of the vascular system in the 

 anodon. In the vertebrates the metamerism is plainer in the 

 embryo (aortic arches) of the higher types. 



The central organ of impulsion of the vascular system — the 

 heart — is to be regarded as a local modification of one of the 

 chief trunks of the system. The simplest form of heart that 

 exists is probably that which occurs in an earthworm (Micro- 

 chceta) ; it is simply a local thickening of the dorsal vessel, 

 which has here more highly developed muscular walls, and is 

 thus possibly more powerfully contractile than the rest of the 



