3/O THE EVOLUTION OF MAN. 



may contain detached cells (lymph-cells) from either fibrous 

 layer. 



A first advance in the development of this most primi- 

 tive vascular system was accomplished by the formation of 

 canals or blood-conducting tubes, which developed, inde- 

 pendently of the cceloma, in the intestinal wall, that is, in 

 the intestinal-fibrous layer of the wall. These real blood- 

 vessels, in the stricter sense, appear in very different 

 form in Worms of the intermediate and higher groups ; 

 sometimes they are very simple, sometimes very complex. 

 Two primordial " primitive vessels " must be regarded as 

 representing that form, which probably formed the first of 

 the more complex vascular system of Vertebrates ; these are 

 a dorsal vessel, which passes from front to back along the 

 middle line of the dorsal wall of the intestine, and a ventral 

 vessel which passes, in the same direction, along the middle 

 line of the ventral wall Both at the front and at the 

 back these two vessels are linked together by a loop sur- 

 rounding the intestines. The blood enclosed in the two 

 tubes is driven forward by the peristaltic contraction of 

 this. 



The further development of this simplest rudimentary 

 blood-vessel system is evident in the class of the Ringed 

 Worms (Annelida), in which we find it in very various 

 stages of development. In the first place, many trans- 

 verse connections probably arose between the dorsal and 

 ventral vessels, so as to encircle the intestine (Fig. 298). 

 Other vessels then penetrated into the body-wall and 

 branched, so as to conduct blood to this part. As in those 

 ancestral Worms, which we have called Chordonia, the 

 front section of the intestine changed into a gill-body, these 



