OF VERTEBRATA. 311 



in the series of transverse fissures (fig. 210, bf) on each side of 

 the neck, and the blood-vessels (ba) which pass between them, 

 when respectively compared with those of the Lancelet (fig. 

 133, br, bo}. On account of this resemblance --in fact by some 

 assumed as an identity both in form and function the fissures 

 have been called by embryologists the branchial fissures^ and the 

 vessels the branchial aorta, the former corresponding with the pas- 

 sages between the gills of fishes, and the latter with the vessels 

 which supply the gills with blood. In respect to the whole blood- 

 system, there are several additions to, as well as modifications of 

 it, beyond what obtained in the last phase. These will be most 

 easily understood by a description of the courses of the vessels 

 as they now exist. The blood passes from the anterior chamber 

 (/<) of the heart in a forward direction, and immediately separates 

 into the four branchial aortas, (ba,} which carry it to the main aorta, 

 (no, ao, 2 ao' 3 ,) that runs along the upper mid-line of the general 

 cavity of the body. Within the latter the blood passes in two 

 directions. One part of it goes forward into the head in what is 

 called the cephalic aorta, (ao,) which, after branching there, unites 

 (at ao 1 ) its scattered currents into two parallel, recurrent vessels 

 (vn) which run on each side of the aorta to a point (at vn l ) near the 

 posterior cavity (h l ) of the heart, and there join that organ at the 

 same place with the vessels which come from behind. The other 

 part runs in the main aorta (ao 2 , ao 3 ) to the waist of the body, 

 and there again divides into two sets of currents. One of these 

 continues direct to the end of the tail in the main aorta (ao 3 ), and 



brain; cr 1 , the middle region of the brain; n, the posterior end of the brain; 

 v, v l , the primitive vertebrae, or vertebra dorsales ; ch, the anterior end, and cA 1 , 

 the posterior end of the chorda dorsalis ; c7* 2 , the chorda dorxalis as seen through 

 the vertebrce dorsales; h, the anterior, (ventricle,') and 7*1, the posterior (auricle) 

 chamber of the heart; oa, the branchial arteries; ao, the cephalic aorta; ao 1 , 

 the end of ao ; ao 2 , ao 3 , the main aorta ; ao 4 , the end of ao 2 , ao 3 , where it joins 

 the afferent vessels (ivz 3 ) ; ef, ef 1 , the main efferent vessel ; e/ 2 , the net-work of 

 efferent vessels ; af, af l , the afferent vessel, or return current, of the vascular 

 area ; vn, vn 1 , the cephalic vein ; im a , vrc 3 , the abdominal vein, or afferent vessel 

 of the right side of the body ; bf, the branchial fissures ; Jl, the right edge of the 

 abdominal aperture ; ys, neck of the nutrient organ. Original. 



