INTESTINAL TEACT OF BIRDS. 257 



cation. Meckel's tract api)ears in two loops separated l)y the diverticulum, the posterior 

 loop having attached to it the long cteca and so forming an incii)i(mt supra-duodenal loop. 

 Underlying the conformations in the other types there is a central pattern which I call 

 the Coraciform-Cucviliform metacentre, and in wliicli the duodciniui is frequently but 

 not invariably wide ; Meckel's tract is produced into two loops anterior to the diverticulum, 

 and a single, ratlier wide snpra-duodenal loop to which the long cteca are attached. Thr 

 Cuculida! retain this metacentral form, only sliglitly altered by increase of length, due 

 no doubt to the relatively large size of these l)irds. It is possible to derive the Psittacine 

 type as a very apocentric modification of this metacentre, the two anterior loops having 

 become enormously long, and often complicated in themselves, the posterior portion of 

 Meckel's tract being produced also into at least two complex loops, the distal of which is 

 a well-specialized supra-duodenal loop, and the ctcca having completely disappeared. 

 However, as the position of the diverticulum is not certain, this derivation of the 

 Psittacine type can be regarded only as provisional. The Striges present a much less 

 apocentric modification of the metacentre. The two anterior loops of Meckel's tract are 

 present, but the second is not so well separated from the general sweep of tlie tract, and 

 appears to be part of a circular expansion of the whole tract. But in some of tin; species 

 of Bilbo the metacentric position is reproduced exactly, and it is easy to regard the 

 conformations in Sfi-Lv, Syrninm, and so forth as very slight alterations of the common 

 form. The Meropida;, the Momotidie, and the Trogoues all retain the metacentric 

 position with extremely little alteration. The LTpupidae, the Alcedinidae, and the Pici 

 are all apocentric modifications of it, the chief difference being the disappearance of 

 the ca^ca and the elaboration of a distinct supra-duodenal loop. 



Summary of the Systematic Description. 



The various conformations of the Intestinal Tract in birds may all be referred to an 

 archecentric form, and this form is well displayed in Valamedea. The Struthious birds 

 (with which the Tinamidse are not included) adhere closely to this archecentric typr. 

 The Turnices and the Galli adhere to it almost as closely, but among the Galli an 

 apocentricity, consisting in the expansion of the distal portion of .Meckel's tract, liegins 

 to appear. The Tinamidae, the Opisthocomidae, and the Pteroclo-Columba; present 

 apocentric conformations which are not easy to derive from those in any other groups, 

 although there are certain suggestions of affinity with the type disjjlayed by Gruiform 

 birds such as Otis. Eor the present, however, it is preferable to regard the forms in 

 these birds as seiiarate, although allied derivatives of the central type. The Passeres 

 again do not show any clear affinity with the types in other birds. The conformation 

 displayed must be regarded as having come separately from an archecentric type, 

 perhaps from that common to tlie other Coraciomorphine birds, but which has 

 progressed very far apocentrically along a radius of its own. The conformations in all 

 other birds may be referred without difficulty to three main metacentres, which are 

 all simple, but different modifications of the archecentric type. The metacentre of all 

 the birds in the Pelargo-Colymbomorphine Brigade is a conformation in wliich the 



