ORIGIN OF BLOOD AND ENDOTHELIUM 79 



that in the phylogenetic origin of the blood vascular system we 

 have the following changes: The primitive condition is r< pre- 

 sented by the development of a space between the body wall. 

 the ectoderm, and the gut wall, the endoderm, that is, the 

 primary body cavity or protocoel. Embryologically the bl 

 coel of the blastular or after gastrulation, the space between the 

 invaginated endoderm and the ectoderm, the schizocoel, repre- 

 sents the primitive vascular space. The body cavity in rotifers, 

 nematodes, bryozoa and arthropods is a primary body cavity 

 and is filled with a fluid, the haemolymph. In the arthropods 

 on the dorsal side of the body is the pulsating heart which sets 

 the fluid in circulation and this fluid contains corpuscles similar 

 to the white blood corpuscles of vertebrates. 



In the arthropods the vessels and heart are often highly devel- 

 oped but all communicate with lacunae and spaces between the 

 gut wall and body wall. The heart is surrounded by a pericardial 

 space (not truly coelomic) which is full of haemolymph, and as 

 the heart pulsates this haemolymph is drawn in through ostia 

 along its walls and then propelled out through the aorta and its 

 arches to the vessels and spaces of the body. These body spaces, 

 or the haemocoele, are thought by some to be a secondary or 

 specialized cavity. Yet it is not coelomic and has no definite 

 lining and resembles very closely the primary body cavity of 

 the rotifers, nematodes, and other invertebrate forms which it 

 most probably represents. In some of the higher Crustacea 

 a secondary body cavity or coelomic space of limited extent is 

 present enclosing the ophthalmic artery in Paelamonetes. The 

 cavities surrounding the gonads are also coelomic, and since 

 these are well developed species the coelomic space here prob- 

 ably represents a progressive rather than a regressive condition. 



The second step in Ziegler's evolution of blood vessels is illus- 

 trated by the conditions in the molluscs. In those animals 

 between the gut and body wall lacunae and interstitial spaces 

 exist which occupy the position of the primary body cavity and 

 these are filled with blood. Vessels lead into the lacunae and 

 the cavities of these vessels as well as the cavity of the well 

 formed heart are also considered to be part of the primary body 



