51S CHORDATA 



arterial trunJc is, at least e.xternalh-, single. The arterial arches show 

 different relations and have different fates. With brancliial respiration 

 the first tliree afferent and efferent arteries are connected in two ways, 

 the one by tlie capillaries of the gills, the other chrect (fig. 569, /'). In the 

 fourth arch there is no gill system, but on the other hand this arch gives 

 off the pulmonary arteries (Jj) to the lungs (compare also lig. 537, II). 



With the loss of gills (fig. 570) the third arch frequently disappears 

 entirely (Anura), as well as the gill circulation of tlie others, while the 

 direct circulation persists, at least in part. The first arch gives rise to the 

 carotids (c) supplying the head, the second unites with its fellow of the 

 opposite side to form the dorsal aorta; the fourth forms the pulmonary 

 artery, and in the Anura, gives oft" a I'litiiiifoiis aiicry (cii) to the skin. A 

 longitudinal fold inside the arterial trunk is so arranged that the venous 

 blood from the body coming to the heart through the right auricle is 

 mostly sent out through the fourth arch to the lungs and the skin, while 

 the blood returned from the lungs by the pulmonary vein passes through 

 the left auricle and then through the first and second arches (carotid and 

 aortic arches). So there is a separation of pulmonary and systemic cir- 

 culations, although the blood all passes through a common ventricle. In the 

 urodeles the primitive part of the last arch, connecting the pulmonary 

 artery with the dorsal aorta, may persist as a Jiidns Bolalli. 



The sexual organs (fig. 530) are similar to those of selachians. The 

 eggs pass from the ovary to the oviducts (Miiller's duct), and in tins are 

 enveloped with a gelatinous layer. The spermatozoa, on the other hand, 

 pass through the anterior part of the Wollhan body ('kidney') and thence 

 out tlirough the ureter. The distinction from the selachians lies in the 

 fact that a urinary bladder, lying ventrally to the rectum, is present, at 

 some distance from the urinary ducts, which open dorsallv into ihe 

 cloaca. Besides sexual organs fat botiics fre(|uenllv occur, lobed and 

 often brightly colored structures, best develojied between the repro- 

 ductive seasons. 



A sort of copulation occurs, and internal impregnation is effected in many 

 urodeles and in the Gymnophiona, but not in the .Vnura. The .\nura and most 

 other forms arc oviparous, but occasionally, a&SaUiDuiinlra iiuu-iilosa and5. nlra 

 of Europe, viviparous species occur. The male of Alvlcs ohslctricaiis wraps 

 the cords of eggs about his legs and crawls into a hole until the young are hatched, 

 while the females of Anipliiidiia and Ichihyophis watch over the eijgs. The 

 male of Rhiiiodcnim Janviiiii has a large sac arising from the pharvn.x in which 

 the esRS and young are cared for until the complerion of the metamorphosis. 

 In Pipa amcricana the male places the eggs on its back, the skin thickening 

 around them so that each lies in a separate pocket, from wliich the voum; escape 

 at Ien!,nh in nearly the adult form. In Xotolrcma and XoloJcIpJivs there are 

 dermal sacs upon the back for the reception of the c^'s. 



