26 VASCULAR SYSTEM 



into an oesophagus, a stomach, and an intestine opening to the 

 exterior by the amis. From the front end of the intestinal region 

 develop glandular outgrowths, the pancreas and the liver. The 

 latter is always a much more complicated structure than the 

 hepatic diverticulum in the Cephalochorda, the lumen of the 

 gland being much subdivided by the formation of an elaborate 

 and compact system of tubules. A specialised sacculation of the 

 duct, the gall-bladder, is present for storing the bile. 



The ventral mesentery is always incomplete, remnants per- 

 sisting in front and behind. A dorsal mesentery (also generally 

 incomplete) supports the alimentary canal, which hangs in the 

 body-cavity. This cavity is continuous, all trace of segmentation 

 having disappeared in the abdominal coelom. In the embryo it is 

 in open communication with the coelom of the branchial segments ; 

 but later a septum is developed cutting off, completely as a rule, 

 an anterior cavity surrounding the heart the pericardium. 

 Abdominal pores, opening from the coelom to the exterior near the 

 anus, are often found in fish, and occasionally in reptiles (Bridge 

 [53], Bles [36]). 



A ventral subintestinal vein in which the blood flows forward ; 

 an anterior prolongation of this vessel in the gill-region (the ventral 

 aorta) ; a dorsal aorta, below the notochord, in which the blood 

 floAvs backward; a system of aortic arches carrying the 

 blood from the ventral to the dorsal aorta through the gill- 

 arches ; a longitudinal latero-dorsal cardinal vein on each side in 

 which the blood converges towards a transverse ductus Cuvieri join- 

 ing the subintestinal vein these are the chief trunks found in the 

 vascular system of Amphioxus (Legros [279ft], Zarnick [511]), and 

 the embryo of all Craniates. In the structure of their blood- 

 vascular system the Craniata have again advanced far beyond the 

 condition found in the Cephalochorda. Not only are the arterial 

 and venous systems much more elaborately developed, especially the 

 capillary networks in the gills, liver, and kidneys ; but also the 

 heart makes its appearance as a special chambered muscular pump- 

 ing qrgan propelling venous blood through the ventral aorta into 

 the gills. It develops as an enlargement of the ventral vein 

 immediately in front of the junction of the ductus Cuvieri with 

 the subintestinal vein. 



The blood-vascular system may communicate, but only in- 

 directly, with the coelom by means of the lymph-holding channels 

 of the lymphatic system, which branch throughout the mesoblastic 

 tissues. Fluid may pass into them through minute stomata in the 

 coelomic epithelium, and be discharged into the blood-vascular 

 system by a few special openings. 



The blood itself consists of a colourless plasma, in which float 

 leucocytes and red haemoglobinous cells or corpuscles. 



