154 Elementary Zoology. 



(the interior of the generative organs and the terminal vesicle 

 of the green gland) is so reduced that all the organs of the body 

 are excluded from it. The hsemocoel differs in these points 

 from the coelom. It is— at any rate, sometimes — formed at first 

 of solid rods of mesoblast (p. 1 60), which melt down centrally, 

 while the marginal cells form the walls. Finer vessels, the 

 capillaries, have been shown in some animals to be produced 

 by the direct canalization of rows of cells. Or, again, blood- 

 vessels sometimes arise first as irregular chinks and lacunse 

 between the mesoblast cells. Blood-vessels are never formed 

 either as archenteric diverticula or as a series of segmented 

 cavities in the mesoblast. Nor do vascular spaces envelop 

 other organs of the body, as does the coelom typically. There 

 appear to be exceptions to this statement, such as the so-called 

 pericardium of Astacus, and the blood sinuses, which have been 

 described as enveloping parts of the alimentary tract in certain 

 worms. As to the crayfish, it has been already pointed out 

 that we have probably to do with a number of veins which 

 have fused together, and thus constitute an envelopment to 

 the ventricle. In the worms referred to, it seems likely that 

 the continuous sinus (if it really exists, a fact which has been 

 questioned) is formed in a similar way by the coalescence of a 

 plexus of blood capillaries. 



The ccelom is further characterized by the fact that it 

 communicates with the exterior, either temporarily (frog) or 

 permanently (earthworm), by means of the excretory organs. 

 It may also have other and more direct communications with 

 the exterior, as in the case of the dorsal pores of the earthworm 

 and the abdominal pores of the dogfish. In the case of the 

 hsemocoel, it is doubtful whether communications with the 

 outside world exist. Capillaries have been stated to open on 

 to the exterior in certain earthworms and in various Amphibia ; 

 but the facts seem to require a renewed investigation. Curiously 

 enough, nephridial funnels do sometimes appear to open into 

 the hsemocoel. Apart from the Nemertine worms, about which 

 there is not an universal consensus of opinion, there is the 

 undoubted fact that in the frog the funnels of the mesonephros, 

 detached from the rest of the organs, create a communication 



