254 THE INTESTINAL TRACT OF BIRDS. 



Dicruridse, Ampelidre, Artamidse, Laniidae, Vireonidse, Sittidse, Oriolida?, Paradiseidse, 

 Corvidge, Stumiidfe, Meliphagidte, Zostei'opid:e, Nectariniidse, Dicaeidae, Coerebidse, 

 Mniotiltidfe, Icterida?, and Pi-ingillida?. This considerable labour from one point of view 

 had a negative result, for I failed to find in tbe characters of the alimentary canal any 

 indication of the families and groups into which systematists have subdivided the 

 Passerines. The differences that exist among them traverse the group irrespectively of 

 the divisions of systematists. These differences depend almost entirely on the size and 

 habits of the birds. The larger forms have a longer gut absolutely and relatively ; the 

 fruit-eaters have the gut extremely short and wide ; grain- and seed-eaters have the 

 gut rather longer. But for these differences the conformation is remarkably similar 

 throughout the group. The duodenum in all is a simple loop, but usually rather wide. 

 The rectum is always short, and the caeca, although present in every case that I have 

 examined, are reduced to short rather nipple-like structures of varying shape, but always 

 with thick walls and only a very small central cavity. Meckel's tract presents the 

 greatest divergences. The diverticulum is missing in the vast majority of cases. As 

 an exception, it is extremely large in Menura, and in a few cases it is present. Relying 

 partly on my own observations and partly on the extended labours of Gadow (12), I do 

 not doubt that the diverticulum in Passeres is situated on tlie apical point of the large 

 portion of Meckel's tract extending from the duodenum to the beginning of the supra- 

 duodenal loop. In birds with a long intestinal tract, as for instance in Corvus capellanus 

 (fig. 72), this portion of Meckel's tract is rolled up into a tight spiral which in the figure 

 is represented as pai'tly drawn out. In ])irds with a shorter tract, as for instance in 

 Cyclorhis albiveiitris (fig. 73), and this may be taken as typical of the vast majority of 

 Passeres, the spiral is represented by two distinct loops, which, in the unfolded 

 condition, are applied very closely to one another, and the folded jjair of loops have a 

 slight spiral twist. In all Passeres there is a supra-duodenal loop very distinctly 

 separated from the rest of Meckel's tract, modelled in the most exact fasliion on the 

 duodenum, and drained by one or more bridging veins from the duodenal vein. The 

 conformation of the gut in the majority of Passeres, although from its short form it falls 

 into a very simple pattern, appears in reality to be a higiily sj)ecialized type, the apparent 

 simj)licity being pseudocentric. In many birds with short guts the intestinal tract is 

 folded in an irregular and almost capricious manner, and the separate portions bear no 

 exact relation to one another. In the Passeres the first two loops of Meckel's tract are 

 most closely applied and extremely similar in caliljre and shape, and their arrangement 

 suggests most strongly that they have been derived by reduction from the proximal 

 portion of what was originally a long spiral. It would apj)ear to be certain that the 

 small Passeres have descended from larger birds. In the same fashion, the supra- 

 duodenal loop is so exactly modelled on the duodenum to which it is applied, as to 

 suggest specialization rather than simplicity. In Passeres generally, the blood-vessels, 

 instead of passing through the centre of the mesenteric expanse, run extremely closely to 

 the intestinal coils, and contribute to the general impression given by the conformation 

 of the gut as a highly specialized type. 



