370 



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 cosloma, 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 diflerent 

 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 dii-ection, 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- 

 verso 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 Chordoma, the 

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



