OF THE KOALA AND VULPINE PHALANGER. 565 



The differences between these two phyllophagous Mammals 

 must therefore be due to complexity of the stomach and the 

 different varieties of specialisation of the interior of the colon 

 and rectum, but the increased internal surface of the colon and 

 rectum of the Sloth is compensated for by the large size and 

 great length of those of the Koala. The Langurs are inter- 

 mediate in the complexity of their intestinal tract between the 

 Sloth and Koala. 



In Phascolomys mitchelli the caecum is considerably reduced, 

 and Lonnberg believes that the colon has become considerably 

 enlarged to take on the cellulose-digesting function of the cfecum. 

 The stomach is simple as in the Koala, so it has not had any 

 modifying influence on the complexity of the intestinal tract 

 as in the Sloth. The Wombat has external sacculi, but the 

 Sloth has internal cavities, so there is a greater similarity 

 between their colons than exists between the colon of either of 

 these animals and that of the Koala. 



I have already shown how the complexity of the stomach is 

 also influenced by the degree of development of the buccal organs 

 (19). One must take a wide view of the whole alimentary canal 

 from mouth to anus in framing an explanation of the physiology 

 of any one part. 



The Ukinary Organs (text-fig. 61). 



PhasGolarctos cinereus. 



The right kidney is 3'2 cm. long, 1*8 cm. v/ide, and 1'6 cm. 

 thick ; it lies entirely anterior to the left Jcidney, whose corre- 

 sponding measurements are 3'7, 2"4, and I'O cm. ; its hilum is 

 ventral in position, whereas that of the left kidney is mesial. 

 The structures in the hilus have the usual disposition in regard 

 to one another. 



On section each kidney is seen to possess a thin cortex, and a 

 single large papilla. 



The surface of each renal pelvis and hilus is covered by a plexus 

 of small veins. These communicate posteriorly with the veins 

 ascending along the ureter, and they open anteriorly into the 

 renal vein. 



The right ihreter lies parallel to, but 1 cm. to the right of, the 

 vena cava inferior. It passes ventral to the iliac vessels and 

 runs posteriorly along the dorsal pelvic wall. It opens into the 

 lower and back part of the bladder. 



It is accompanied by small veins from the pelvis ; the latter 

 are joined by veins from the ovaries, after which a ladder-like 

 arrangement of veins runs up to the venous plexus on the ventral 

 surface of the renal pelvis (text-fig. 61 C, lov). 



The left ureter runs along the left border of the abdominal 

 aorta, crosses the ilin.c vessels, and courses through the pelvis as 

 does the right one. The veins accompanying it are numerous 



