RUDIMENTARY VASCULAR SYSTEM. 369 



begin till a comparatively late period. Not only all Plant 

 Animals (Sponges, Corals, Hydropolyps, Medusse), but also 

 all lower Worms (Accelomi), are entirely destitute of 

 vascular system. In both groups, the fluid acquired by 

 digestion is conveyed directly from the intestinal tube, 

 through processes of this latter (the gastro-canals), into the 

 different parts of the body. It is only in the intermediate 

 and higher Worms that the vascular system first begins to 

 develop, in consequence of the formation of a simple cavity 

 (cceloma), or of a system of connected spaces, round the 

 intestinal tube, in which cavities the nutritive fluid (blood) 

 exuded through the intestinal wall, collects. 



In the human ancestral line we meet with this first 

 rudiment of the vascular system in that group of Worms 

 which we spoke of as Soft Worms (Scolecida; p. 85). 

 The Soft Worms, as we said, formed a series of intermediate 

 stages bstween the lowest bloodless Primitive Worms 

 (Archelminthes) and the Chorda-worms (Chordonia), which 

 are already provided with a vascular system and a noto- 

 chord. The vascular system must have begun, in the older 

 Scolecida, with a very simple coelom, a " body-cavity," 

 filled with blood, and which surrounded the intestinal tube. 

 Its origin was probably due to the accumulation of 

 nutritive fluid in a cleft between the intestinal-fibrous 

 layer and the skin-fibrous layer. A vascular system in 

 this simplest form is yet found in the Moss-polyps (Bryozoa) 

 in the Wheel-animalcule (Rotatoria), and in other lower 

 Worms. The inner, visceral, part of the wall of the ccelom 

 is, naturally, formed by the intestinal-fibrous layer (endo- 

 ccelar), the outer, parietal, part by the skin-fibrous layer 

 (exocalar). The ccelom fluid, collected between the two, 



