VASCULAR SYSTEM OF VERTEBEATA. 575 



Vascular System. 

 § 429. 



The nutrient fluid of tlie Vertebrata moves along closed canals, 

 wliicli have proper walls ; it is very rarely that this tract becomes 

 lacunar in character. In this point it differs from the arrangements 

 which obtain in the MoUusca, though it is more nearly similar to those 

 of the Vermes. Its cavities form a system of canals — a vascular 

 system. This portion is derived from the mesoderm, which also 

 develops the parts that carry the vessels. The chief trunks are 

 placed in the middle line, and give off branches in accordance with 

 the segmentation of the body ; looked at in the broadest way, the 

 arrangement of these vessels calls to mind many arrangements which 

 obtain in the Invertebrata. This may be still more distinctly seen 

 in the relations of the longitudinal trunks to the respiratory portion 

 of the enteric canal. But there is a great difference when a central 

 organ of circulation is developed. In the Arthropoda, Mollusca, 

 and most Vermes, this is developed from a dorsal vessel, or from 

 part of one, but in the Vertebrata it is formed from a ventral 

 portion. The double character of the early rudiment of the heart, 

 such as has been made out in the higher Vertebrata (Rabbit), 

 cannot be referred to any definite vascular organ, for none of the 

 kind is known to us. 



There are considerable differences between the circulatory centres 

 of the nutrient fluid in the two great groups of the Vertebrata, 

 so that we must separate the apparatus which obtains in Amphioxus 

 from that of the Craniota. In the former, all the larger vascular 

 trunks are contractile, and so call to mind the arrangements which 

 obtain in the Vermes. The contents of the vascular system are 

 driven on at various points, without one centre being at all aided by 

 the others. The following is a description of the arrangements of 

 these vessels. Below the respiratory portion of the enteric canal there 

 is a longitudinal canal, which gives off branches, at regular distances, 

 to the branchial framework; these branches are branchial arteries. 

 They are collected into a trunk, which is placed above the gills — the 

 aorta ; and from this they are distributed through the body. At the 

 root of each branchial arterv there is a heart-like formation, or con- 

 tractile enlargement. The most anterior pair of branchial arteries 

 passes into two arches, which are likewise contractile, and which sur- 

 round the mouth j this pair is connected with the commencement 

 of the aorta. From this vascular trunk arterial blood-vessels are 

 distributed in the body. 



Our knowledge of the vessels, which distribute the blood, is at 

 present very slight, while the afferent vessels also require to be 

 more accurately investigated. It seems to be certain, however, that 

 there are contractile tracts in this portion also of the vascular 



