OCR Text |
Show 1896.] INTESTINAL TRACT OF BIRDS. 155 loop of the mid-gut with its special vein apparently being a reminiscence of the stage with functional caeca. In the Parrots, Macaws, and Parrakeets that I have examined the gut presents no great divergences. It is invariably very long and slender, and the subsidiary loops are folded upon each other, and twisted and doubled in a very perplexing manner. Moreover, the masses of twisted gut are overgrown by connective tissue loaded with fat, and short-circuiting connections between the veins are common. The relation to the common type, however, is easily made out. Ara ararauna (fig. 21) may serve as an instance; the Fig. 21. Ara ararauna ; intestinal tract, x, short-circuiting vessel divided. duodenum is considerably wider than the rest of the gut, and is a simple loop, partly curved at the end. The circular loop is enormously expanded and is pulled out into a number of subsidiary loops, four in number, as in Ara, but numerous minor subsidiary loops usually occur between them. The first of the four is short in Ara; the second, as in the others that I have examined, bears the vestige of the yolk-duct at its extremity; the third and fourth are very long, and the fourth has a short-circuiting vein to the duodenum, and corresponds to the part of the circular loop along which the caeca run in the primitive type. The rectum is straight and bears no trace of caeca. The three main veins-the duodenal, the median, and the posterior mesenteric-occur in the typical fashion. W h e n the minor loops between the four subsidiary loops are abundant, as, for instance, in Chryosotis, the gut bears a resemblance |