632 THE VASCULAR SYSTEM. 



become isolated from the remainder of the body or peritoneal 

 cavity. As shewn by their development the so-called pleurae or 

 pleural sacks are simply the peritoneal linings of the anterior 

 divisions of the body cavity, shut off from the remainder of 

 the body cavity by the diaphragm. 



The exact mode of formation of the diaphragm is not fully 

 made out ; the account of it recently given by Cadiat (No. 491) 

 not being in my opinion completely satisfactory. 



BIBLIOGRAPHY. 



(491) M. Cadiat. " Du developpement de la partie cephalothoracique de 1'em- 

 bryon, de la formation du diaphragme, des pleures, du pericarde, du pharynx et de 

 1'cesophage." Jotirnal de P Anatomic et de la Physiologic, Vol. xiv. 1878. 



Vascular System. 



The actual observations bearing on the origin of the vascular 

 system, using the term to include the lymphatic system, are 

 very scanty. It seems probable, mainly it must be admitted on 

 a priori grounds, that vascular and lymphatic systems have 

 originated from the conversion of indefinite spaces, primitively 

 situated in the general connective tissue, into definite channels. 

 It is quite certain that vascular systems have arisen indepen- 

 dently in many types ; a very striking case of the kind being 

 the development in certain parasitic Copepoda of a closed 

 system of vessels with a red non-corpusculated blood (E. van 

 Beneden, Heider), not found in any other Crustacea. Parts of 

 vascular systems appear to have arisen in some cases by a 

 canalization of cells. 



The blood systems may either be closed or communicate 

 with the body cavity. In cases where the primitive body cavity 

 is atrophied or partially broken up into separate compartments 

 (Insecta, Mollusca, Discophora, etc.) a free communication 

 between the vascular system and the body cavity is usually 

 present; but in these cases the communication is no doubt 

 secondary. On the whole it would seem probable that the 

 vascular system has in most instances arisen independently of 

 the body cavity, at least in types where the body cavity is 



