IX EXTEROPXEUSTA— BLOOD VASCULAR SYSTEM 581 



tongue, immediately behind the septum. Each fork thus has a 

 median septal prong, and an anterior and a posterior lingual prong. 

 Each tongue has two prongs, one anterior and the other posterior, 

 but these belong to two different forks. Each septum has only one 

 prong. A very minute examination shows, however, that each septal 

 prong consists of two fused prongs. Two-pronged forks must, there- 

 fore, be the ultimate elements of the branchial skeleton. Each fork 

 would lie with one prong in a tongue and the other in a septum. 

 The two septal prongs belonging to two consecutive two-pronged 

 forks, are, however, in every case fused together. 



The most anterior skeletal fork, and it alone, has two prongs. 



In the formation of the lingual prongs, the branchial epithelium 

 (belonging to the intestine) and the mesodermal, inner wall of the 

 lingual cavity (belonging to the visceral wall of the trunk coeloni) 

 take part, but the septal prongs are secreted exclusively by the 

 branchial epithelium of the septal edge. 



IX. The Blood Vascular System. 



The blood vascular system consists of spaces in the limiting mem- 

 branes of the body. The two lamellae of the limiting membranes 

 simply remain apart at certain points, thus forming the walls of 

 the vessels. An endothelium-like covering of the inner side of the 

 separating lamellee has only been found in Ptyclwdera, and in isolated 

 parts in Schizocardium and Glaiuliceps. Nothing of the sort has been 

 observed in Balanoglossus. In Ftychodera, isolated blood cells float in 

 the colourless blood fluid. 



The lacunar blood vessels of the Enteropneusta do not arise by 

 the 'separation of the formerly contiguous lamellae of a limiting 

 membrane. They are, rather, persistent portions of the larval 

 segmentation cavity or blastocoel. The organs of the larva lie in 

 a spacious blastoccel, which narrows and disappears in proportion as 

 the organs (especially the ccelomio sacs) increase in size, these 

 swelling up in such a way that their walls come in contact with one 

 another and with the body and intestinal epithelium. Certain 

 cavities, however, persist, which afterwards form the blood vascular 

 system. The blood cells and endothelial cells of the vascular system 

 are, in all cases, of mesenchymatous origin. 



The arrangement of the vascular system may be roughly described 

 as follows. 



There is a capillary network in all the limiting membranes of 

 the body, especially in that of the integument and of the intestine. 

 This network is in connection with larger vessels, i.e. (1) with a dorsal 

 vessel which, in the dorsal mesentery, runs through the trunk and 

 collar and communicates with the blood vessels of the proboscis, and 

 (2) with a ventral vessel which, running in the ventral mesentery of 



