PHYLUM VERTEBRATA (CRANIATA). 49 



The vascular system is well developed, consists of arteries, 

 capillaries and veins, and contains a red blood. There is a 

 median ventral subpharyngeal vessel, the hinder end of which 

 is especially muscular and contractile and differentiated as 

 the heart. The lymphatic system consists of vein-like 

 vessels containing a colourless fluid — the lymph — in which 

 float numerous amoeboid cells (lymph corpuscles). These 

 vessels commence by blindly-ending fine tubes or sinuses in 

 the tissues, which gradually unite with one another to form 

 the main lymph vessels, which open into the venous system. 

 Special gland-like bodies, the so-called lymphatic glands, in 

 which the lymph corpuscles are produced, are inserted in the 

 course of the lymphatic vessels. The lymphatic system is a 

 draining system, for the purpose of carrying away from the 

 tissues the fluid which has exuded into them through the walls 

 of the blood-capillaries, and is undoubtedly a specially differ- 

 entiated i^art of the vascular system. 



The body-cavity is a coelom, and has the usual relations 

 of that organ to the urinary and reproductive organs. It is 

 laid down early, making its appearance as a split (schizocoel) 

 in the mesoblast ; and in the Elasmobranchii, at any rate, a 

 certain resemblance between it at its first appearance and that 

 of Amphioxus can be detected (p. 33). But it differs from 

 that of Amphioxus in that the ventral portions of the trunk 

 somites are never distinct from one another, but form from 

 the first a continuous splanclmocoel. In the adult the 

 body-cavity is always divided more or less completely into 

 a pericardial division in front and a peritoneal division or general 

 body-cavity behind. In the mammals the latter is still further 

 subdivided, in that two anterior horns are cut off from it to 

 form the pleural cavities. There is no coelom in the head of 

 adult vertebrata. 



The urinary organs consist typically, in their origin at 

 least, of segmentally arranged nephridia, which open internally 

 into the body-cavity. Externally they open into a longitudinal 

 duct which leads into the hind end of the intestine in almost 

 all cases. Both nephridia and ducts develop as special portions 

 of the coelom. 



The generative organs develop from the lining of the 

 unsegmented ventral part of the coelom (splanclmocoel), and 



