420 Vertebrata. 



It is not, however, single, as in the Pisces and some Amphibia j 

 but is divided into three vessels derived from three pairs of 

 arterial arches, viz., 1, 2, and 4. The first of these vessels 

 forms the carotids (arterial arch 1) and the right aortic arch (the- 

 right arch of the second pair) ; the second is prolonged into the- 

 left aortic arch (the left arch of the second pair), whilst the third 

 forms the pulmonary arteries (arterial arch 4). At the origin of 

 each vessel there is a transverse row of valves. The carotids and 

 the right aortic arch arise from the left ventricle and carry arterial 

 blood, whilst the pulmonaries and the left aortic arch are given 

 off from the right ventricle and carry venous blood. In conse- 

 quence of this arrangement, the head is supplied with pure blood, 

 whilst the aorta carries mixed blood, for it is formed of the union of 

 two arches, of which one contains arterial, the other venous, blood. 



In tlie Crocodilia, the larger portion of venous blood from the left aortic arch goes 

 direct to the alimentary canal, by a vessel (m) -which it gives ofB before it unites 

 •with the right aortic ai-oh, so that in the aoi-ta arterial blood preponderates.* In 

 other Reptiles, in -which the ventricular septum is quite imperfect, the blood 

 is noixed -within the heart itself, but here various arrangements prevent th& 

 mixing from being as complete as might be expected. The whole mechanism, 

 is too complicated to receive more detailed consideration hei-e. 



The kidneys are somewhat elongate, lobulated organs, lying 

 posteriorly in the body-cavity ; the urinary tubules have no open 

 funnels as in the Amphibia. The ureters discharge separately into 

 the cloaca, not into the urinary bladder. There is a urinary bladder 

 in Lacertilia and Chelonia, not in Ophidia or Crocodilia; it is an 

 evagination of the ventral wall of the cloacat; the openings of the 

 ureters are not far from the opening of the bladder. 



The two ovaries are racemose when ripe in consequence of the 

 large size of the ova; the oviducts (Miiller's ducts) are of the ordinary- 

 type, and open separately into the cloaca. In Snakes, in correspond- 

 ence with the elongation of the body, the ovaries lie one in front 

 of the other. The testis is connected, by an epididymis of fine- 

 tubules, with a sperm duct which opens into the cloaca. C o p u - 

 latory organs of two kinds are met "with. In the Lacertilia 

 and Ophidia there is a p a i r of these organs ; on each side, 

 quite close to the anus, there is an opening leading into a sac or 

 tube, which extends back below the skin of the tail, and is to be- 

 regarded as an invagination of the skin ; this sac can be everted, and 

 then displays on its surface a spiral groove, along which the sperm 

 can travel when the organ (which is often provided with spines or 



* The right arterial arch (and the carotids) coimnunioates -with the left arterial arch 

 by means of an opening in the septum between the two vascular trunks ; mixing of 

 the blood, ho-wever, occurs only to a very limi-ted extent at this spot. 



t In the Chelonia, besides the unpaired urinary bladder, a pair of similar sacs of 

 unknown significance opens into the cloaca. 



