384 THE EVOLUTION OF MAN. 



distinction between the exoccelar, or the parietal coelom- 

 epithelium, which originates Irom the skin-fibrous layer, and 

 the endoccelar, or the visceral ccelom-epithelium, which 

 is derived from the intestinal-fibrous layer. The former 

 is, perhaps, connected with the male germ-epithelium (the 

 rudiment of the testes), the latter with the female germ- 

 epithelium (the rudiment of the ovary). (Of. Chapter XXV.) 



TABLE XL. 



SYSTEMATIC SURVEY OP THE MOST IMPORTANT PERIODS IN THE PHYLOGENT 

 OF THE HUMAN VASCULAR SYSTEM. 



I. First Period : Vascular System of the earlier Scolecida. 

 Between the skin-covering and the intestinal wall is formed a simple 

 body-cavity (cceloma), or a perienteric cavity (as in the extant Bryozoa and 

 other Ccelomati). 



II. Second Period : Vascular System of the more recent Scolecida. 

 The first real blood-vessels form in the intestinal wall (in the intestinal- 

 fibrous layer), a dorsal vessel in the central line of the dorsal side of the 

 intestinal tube, and a ventral vessel in the central line of its ventral side. 

 The two vessels are connected by several circular vessels, encircling the 

 intestine. 



III. Third Period : Vascular System of the earlier Chordonia. 



By the modification of the anterior half of the intestine into a gill- 

 intestine, the anterior section of the ventral vessel becomes a gill-artery, 

 and the anterior section of the dorsal vessel a gill-vein ; between the two 

 a gill capillary network develops. 



IV. Fourth Period : Vascular System of the more recent Chordonia, 

 The portion of the ventral vessel, lying immediately behind the gill. 

 intostine, enlarges to a simple heart-pouch (Ascidian). 



