216 BULLETIN OF THE 



tainly allows the inference that it is a reduced primitive dorsal aorta, — 

 the exact horaologue of the dorsal aorta of any vertebrate possessing the 

 precardiac section. 



In studying the course of the blood in the vessels of Chlamydosela- 

 chus, we find that the complete ellipse formed by each efferent branchial 

 artery in the majority of Elasmobranch* species is wanting, and a 

 single trunk collects the blood from all the gill leaflets borne by an arch, 

 and consequently from one side only of any given gill sack. This is the 

 primitive condition, and from Dohrn's researches we know that it is 

 entirely in agreement with the embryonic structure of most of the 

 Teleost and Elasmobranch embryos studied. It also agrees with the 

 adult condition of Amphioxus. Parker very justly takes exceptions 

 to the current use of the term " branchial vein," as applied to an 

 efferent branchial vessel, and I quite agree with him when he says 

 respecting the nature of these arteries (loc. cit., p. 688) : "These vessels 

 are usually, but very incorrectly, called branchial veins. It would be 

 quite as justifiable to speak of the portal artery as to call these obviously 

 arterial vessels veins ; a capillary system may be interposed in the 

 course of an artery or of a vein, but this does not make the efferent 

 trunk in the one case a vein, nor in the other the afferent trunk an 

 artery." The collecting trunk is continued uninterruptedly to the dor- 

 sal aorta, so that an epibranchial artery in Parker's sense is not present 

 in Chlamydoselachus. He says (loc. cit., p. 689) : " Prom the dorsal end 

 of each arterial loop an epibranchial artery is continued backwards 

 and inwards (Fig. 11) ; by uniting with one another successively in 

 pairs these four trunks form the dorsal aorta. ... In the embryo the 

 aortic arches are continued directly from the ventral to the dorsal aorta. 

 In the Holocephali and Teleostei there is only one efferent artery to 

 each gill, corresponding to the anterior of the two efferent arteries in 

 the Plagiostome holobranch. This is very evident in Callorhynchus, in 

 which the single efferent artery of each gill is always cephalad of the 

 corresponding afferent trunk. These facts tend to confirm the opinion 

 to which one is led by the simple inspection of the parts in the adult 

 Mustelus (compare loc. cit., Figs. 6 and 17) ; namely, that the anterior 

 efferent artery of each holobranch is to be looked upon as its primary 

 revehent trunk and as strictly continuous with the corresponding epi- 

 branchial artery, the posterior efferent artery being a secondary vessel 

 which debouches not into the primary trunk of its own, but into that of 

 the next following gill." Such are Parker's conclusions from the study 

 * Cf. Hyrtl, loc. cit., p. 4, and Parker (4) and (7). 



