ARTERIAL SYSTEM. 645 



branchial arteries bringing the blood to the gills from the truncus 

 arteriosus, and the branchial veins transporting it to the dorsal 

 aorta. The branchial vessels to those arches which do not bear 

 gills, either wholly or partially atrophy; thus in Elasmobranchii 

 the mandibular trunk, which is fully developed in the embryo 

 (fig- T 93> la " c '\ atrophies, except for a small remnant bringing 

 blood to the rudimentary gill of the spiracle from the branchial 

 vein of the hyoid arch. In Ganoids the mandibular artery 

 atrophies, but the hyoid is usually preserved. In Teleostei both 

 mandibular 1 and hyoid arteries are absent in the adult, except 

 that there is usually left a rudiment of the hyoid, supplying the 

 pseudobranch, which is similar to the rudiment of the mandibular 

 artery in Elasmobranchii. In Dipnoi the mandibular artery 

 atrophies, but the hyoid is sometimes preserved (Protopterus), 

 and sometimes lost. 



In Fishes provided with a well developed air-bladder this 

 organ receives arteries, which arise sometimes from the dorsal 

 aorta, sometimes from the casliac arteries, and sometimes from 

 the dorsal section of the last (fourth) branchial trunk. The 

 latter origin is found in Polypterus and Amia, and seems to have 

 been inherited by the Dipnoi where the air-bladder forms a true 

 lung. 



The pulmonary artery of all the air-breathing Verte- 

 brata is derived from the pulmonary artery of the 

 Dipnoi. 



In all the types above Fishes considerable changes are 

 effected in the primitive arrangement of the arteries in the 

 visceral arches. 



In Amphibia the piscine condition is most nearly retained 2 . 

 The mandibular artery is never developed, and the hyoid artery 

 is imperfect, being only connected with the cephalic vessels and 

 never directly joining the dorsal aorta. It is moreover developed 

 later than the arteries of the true branchial arches behind. The 

 subclavian arteries spring from the common trunks which unite 

 to form the dorsal aorta. 



In the Urodela there are developed, in addition to the hyoid, 



1 The mandibular artery is stated by Gotte never to be developed in Teleostei, but 

 is distinctly figured in Lereboullet (No. 71). 



2 In my account of the Amphibia, Gotte (No. 2'J6) has been followed. 



