MUSEUM OF COMPARATIVE ZOOLOGY. 199 



has then, in all, nine aortic arches and remnants of arches, persisting 

 from an earlier and more primitive condition of organization. This 

 count is based on the demonstrations and suppositions, (a) that the 

 anastomotic branch between the hyoidean demibranch and first efferent 

 branchial is the continuation of the hyoidean efferent artery ; (b) that 

 the internal carotids after turning inwards and entering the pituitary 

 space unite with the dorsal aorta ; (c) that the anterior end of the cranial 

 aorta divides, and that the lateral vessels curve outward each side of 

 the anterior end of the notochord ; (d) that the efferent artery of the 

 spiracular pseudobranch is connected with the aorta ; and (e) that the 

 sixth efferent branchial artery formed at one time (probably during 

 embryonic life) an independent aortic root. The fact should not be 

 overlooked that we thus find remnants of two, and perhaps three, aortic 

 arches in the pituitary space/ (See table on page 218, and Figs. 1 

 and 2.) That the above method of counting the aortic arches in 

 Chlamydoselachus is a correct one, within very narrow limits of error, 

 no one will question who recognizes the vast changes that have been 

 brought about in the vascular system of a higher vertebrate — mammal, 

 for instance — during its phylogenetic course from the fish type up- 

 wards, and who recognizes the general law that an organism makes use 

 of rudimentary or disused structures to build up other structures, of 

 different function perhaps, for use under changed conditions of environ- 

 ment, provided the rudiment or disused structure be suitably placed. 

 The only portions of the vascular apparatus of the branchial region that 

 are suitably placed for use in case the aorta is reduced, are evidently the 

 dorsal and ventral commissural systems. That the dorsal vessel should 

 be chosen of the two is further evidence of the law, for the dorsal 

 commissure is both more directly connected with the territory to be 

 supplied, and lies deeper in the tissues in a direct line toward the 

 brain ; besides, it normally carries the purified blood from the gills, 

 which the ventral commissures do not to so great an extent, lying as 

 they do on the side of the gills where the currents are forming and 

 setting towards the dorsal vessel. As a further illustration, we find in 

 some species of Myxine the remnant of a ductus Botalli. This remnant 

 was in early adult life hollow, and connected the gills of its half of the 

 segment with the dorsal aorta. Miiller found in some cases that each 

 end of the thread-like remnant was still hollow. These threads arise 

 from the afferent branchial artery of the anterior gill sac, passing 

 thence upwards and forwards, and fuse with the carotid trunks where 

 the latter anastomose with the first efferent branchial arteries. In 



