620 mh. r. oiialmbrs Mitchell on tue [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 ver}' rough spiral. In 

 the chicle and in two udults 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 A'estige 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 caBca 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 OpistJioconnis 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 mid- 

 gut, from the duodenum to the insertion of the long cseca, 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 Gallidio 

 and CracidoD, and, among aquintocubital birds, in Clumnu and 

 Falamedea, in Jlimantojnis, Giarenla, and Cupnnmlgus. So far 

 as I have had opportunity of examining them, and I have now 

 more than doubled the material upon which I first formed 

 ihe 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 ])rimitive 

 circular loop, while the direction of the divergences is, on the whole, 

 identical in each group. O^nstJwconius, inasmuch as its mid-gut 

 displays differentiation into three woll-niarked subsidiary loops, 

 has advanced beyond the Gallidio, Cracidaj, and iStruthious birds. 

 Its mode of divergence differs from that of the Tinamou, in which 

 the first and third subsidiary loops are verj^ long, but in which the 

 region bearing the yolk-sac vestige and corresponding to the median 

 loop is not expanded. Neglecting the fact that Fterocles and the 

 Pigeons are aquintocubital. while Ojjisthocomns is certainly quiuto- 

 cubital, the latter from the form of the gut is intermediate between 

 Pterodes and the Pigeons. In these three the mid-gut has three 

 loops, the central loop bearing the yolk-sac vestige : as in Plerocles 

 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 Opisihocomus shows the closest resemblance to 

 the Cuculida), in which also the ca3ca are long and the mid-gut is 

 thrown into three loops, the median loop beariug the yolk-sac 

 vestige. So far as argument may be based upon the formation of 

 the mid-gut, either Huxley's ' suggested relationship between 



' "On the Classiflcution and Disti'ibuliou of the Alcctoromoriiliie unit 

 HeteroMiorjjha;," V. Z. S. 18US, p. 2t)J. 



