OCR Text |
Show 620 MR. P. CHALMERS MITCHELL ON THE [June 16 The mid-gut is thrown into three well-marked loops : the first of these is long and narrow; the second is long, is much more open and shows a tendency to be thrown into a very rough spiral. In the chick and in two adults I found no trace of the yolk-sac diverticulum, but its place of attachment was marked by a distinct and strong remnant of the ventral mesentery; in a third adult as shown in the figure, this mesentery ran to a minute vestige of the yolk-sac, placed nearly at the summit of the middle loop. The third loop of the mid-gut is wide, and along it the caeca run in the fashion characteristic of birds in which these are functional; where the duodenum lay over this third loop, a bridging vein ran from the caeca to the duodenal branch of the mesenteric vein. The rectum is very long and is thrown into secondary folds. It is obvious that the gut of Opisthocomus exhibits a definite divergence of a simple nature from what I tried to show, in the paper referred to above, to be the primitive type of avian intestines. The chief character of the typical intestinal folds is that the midgut, from the duodenum to the insertion of the long caeca, is a simple loop, thrown into short folds at the circumference of an almost circular expansion of the mesentery, and bearing near its median point a vestige of the yolk-sac. Such a condition occurs almost unmodified in the Struthious birds, in the Gallkke and Cracidae, and, among aquintocubital birds, in Chauna and Palamedea, in Himantopus, Glareola, and Caprimulgus. So far as I have had opportunity of examining them, and I have now more than doubled the material upon which I first formed the conclusion, nearly every group of birds contains members approaching this primitive type. The divergences consist in the stretching out and twisting of secondary loops of this primitive circular loop, while the direction of the divergences is, on the whole, identical in each group. Opisthocomus, inasmuch as its mid-gut displays differentiation into three well-marked subsidiary loops, has advanced beyond the Gallidae, Cracidae, and Struthious buds. Its mode of divergence differs from that of the Tinamou, in which the first and third subsidiary loops are very long, but in which the region bearing tbe yolk-sac vestige and corresponding to the median loop is not expanded. Neglecting the fact that Pterocles and the Pigeons are aquintocubital, while Opisthocomus is certainly quin to-cubital, the latter from the form of the gut is intermediate between Pterocles and the Pigeons. In these three the mid-gut has three loops, the central loop bearing tbe yolk-sac vestige: as in Pterocles the ca-ca are long ; the middle loop shows a trace of the spiral formation which is characteristic of the higher Pigeons. Among quintocubital birds Opisthocomus shows the closest resemblance to the Cuculidae, in which also the caeca are long aud the mid-gut is thrown into three loops, the median loop bearing the yolk-sac vestige. So far as argument may be based upon the formation ot the mid-gut, either Huxley's' suggested relationship between 1 " O n the Classification and Distribution of the Alectoromorphse and Heteromorphae," P. Z. S. 1868, p. 294. |