298 ZOOLOGY SECT. 



lungs appear, a pulmonary artery (pul. a.) is given off from the 

 dorsal portion of the fourth aortic arch of each side. In those 

 Urodela which in the adult condition are devoid of gills, when the 

 latter atrophy (B) the first aortic arch loses its connection with the 

 dorsal aorta, and becomes the carotid trunk ; the second increases 

 in size, forming the main factor of the dorsal aorta, and becomes 

 the systemic trunk ; the third undergoes great reduction, and the 

 fourth becomes the pulmonary artery, its dorsal portion retaining 

 its connection with the systemic trunk in the form of a small con- 

 necting branch, the ductus Botalli (d. bot.). In the Anura, as we 

 have seen (p. 271), the third arch vanishes completely and there is 

 no ductus Botalli. 



As to the venous system, the Urodela exhibit very clearly the 

 transition from the Fish-type to the condition already described 

 in the Frog. The blood from the tail is brought back by a caudal 

 vein (Fig. 965, Gaud. V.), which, on reaching the coelome, divides into 

 two renal portal veins, one going to each kidney. From the kidney 

 the blood is taken, in the larva, into paired cardinal veins, each of 

 which joins with the corresponding jugular to form a precaval vein. 

 In the adult the anterior portions of the cardinals undergo partial 

 atrophy, becoming reduced to two small azygos veins (Card, post.) 

 which receive the blood from the region of the back : their posterior 

 portions unite and are continued forwards by a new unpaired vein, 

 the postcaval (V. cava inf.), which, joined by the hepatic veins, 

 pours its blood into the sinus venosus. The iliac vein from the 

 hind-leg divides into two branches : one joins the renal portal, 

 the other, representing the lateral vein of Elasmobranchs, unites 

 with its fellow in the middle ventral line to form the abdominal 

 vein (Abd. V.) and joins the hepatic portal, its blood, after 

 traversing the capillaries of the liver, being returned by the hepatic 

 vein into the post-caval. 



The red corpuscles are oval and nucleated, and are remarkable 

 for their unusual size. Those of Amphiuma are the largest known, 

 being about T T T mm. in diameter, or eight times that of a human 

 red corpuscle. 



Nervous System and Sense-Organs . The brain of Urodela 

 differs from that of the Frog in its more elongated and slender 

 form, in the comparatively small size of the optic lobes, and in the 

 non-union of the olfactory bulbs. The olfactory sacs always open 

 into the mouth by posterior nares situated behind or external to 

 the vomers. The eye has no lids in the lower forms and is de- 

 generate in the cave-dwelling Proteus and in some Gymnophiona. 

 The Urodela, the Gymnophiona, and some Anura have no tympanic 

 cavity or membrane, and no columella ; there is, however, a stapes 

 (Figs. 959, 960) in the form of a nodule of cartilage inserted in the 

 fenestra ovalis. In the perennibranchiate Urodeles and in the 

 larvae of the air-breathing forms lateral-line sense-organs are 



