4S-i 



CHORPATA. 



\-cins. rousistint; of Ciivicrlaii ilncts and iiii^ular and cardinal veins, tlie 

 laucr with growth eiicroacliiiiy; more aiul more into the territory ol lire 

 suhiiiteslinal veiir. 



The eirculatioir of the lish tvpe umlerL;oes a great moililK-atioii with the 

 loss of gills and the appearanee of pidnioiiarv respiration in the higher 

 vertebrates. Gills and gill eapillaries disappear, aird the hranehial 

 cireulaliori is reduced to arlcrial arclics leading direct from (he \-entral to 

 the dorsal aorta The swim Idadder recei^■ed its Mood froni the hody 

 (s}-stemie~) circulation, luit with the functioning of the lungs pulmonary 

 arteries and veins come into existence, while the arterial arches in part 



Fig. 537. — Diagram of moditication of arterial arclics in \-arious \-cricbr,uc classes. 

 W'liitc, \*essels wliich clcgencrale; cross-lhioil, ^■csscls coiUainiiig arlcrial liU>i.>>l; tUacI^. 

 vessels c^tntainiiig \cnoiis I'lood. /, Uijinoi: //, t'riiilclcs willi |niliiionar\" rcs]tiralion ; 

 ///, Reptiles; /I', Birds un inaiuiiials the left iuslcad of Uic riglit aortic arcli jicrsisls^. 

 ao\ venous aorta of reptiles; ao'-, arterial aorta; J.v7. arterial trunk; a. />, arches which 

 usually disappear; ad, dorsal aorta; d.B. ductus Botalli; k, gill capillaries; />», pulmo- 

 nary artery; 1-4, persistent arterial arches. 



disappear, in part arc divided between the pulmonary and svstemic 

 circulations (hg. 537). Of the six arches which usually appear in the 

 emliryo, tlie first and secoird, atid tlie fiftli in airimal-e with lungs, usually 

 degenerate. Tlie last arch (4), which e\en in the Dipnoi sujiplies the 

 swim bladder, becomes a [>uhnonary aiicry, the other arches (i and 2) 

 furnish the systemic portions — (he dorsal aorta (2] and the carotids supplv- 

 ing the head (i). Since special pulmonary veins, distinct from the svstemic 

 circulation, carry the blood from the lungs to the heart, the heart be- 

 comes divided by a septum which separates il into right and left hahes. 

 The right half retains the \eirous character of the lish heart; since the 

 right auricle receives the systemic veins, the right \entricle gives oil the 

 pulmonary artery. The left half is purely arterial, receiving arterial 

 blood by the left auricle from the lungs and sending it out through 

 the aorta ascendens to the l)ody. This complete separation of pul- 

 monary and s}-stemic circulation, and the corresponding di\ision of 



