258 



DE. P. CHALMEKS MITCHELL ON THE 



symmetry of Meckel's tract about the median mesenteric vein and the Meckel's 

 diverticulum is retained, but in which Meckel's tract tends to be produced into a 

 number of long and narrow loops, the most distal of which is a supra-duodenal loop 

 to which the long ceeca are attached. The metacentre of the Limicoline-Gruiforrn 

 assemblages (which contains the Alectoromorphine Legion without the Tinamiformes, 

 Galliformes, and Pteroclo-Columbae) is a derivative of the archecentre in which Meckel's 

 tract is no longer symmetrical about the middle mesenteric vein and the diverticulum, 

 but in which the region anterior to the diverticulum is produced beyond it so as to form 

 an axial loop, on the distal limb of which the diverticulum lies and which may be greatly 



Fig. 75. 



Pas seres 



Coracimorphine 

 Metacentre 



Pelargo- 



Colymbimorphme 



Metacentre 



Limicoli- 



Gruiform 



Metacentre 



Pteroclo- Columbas 

 Opisthocomi 



Tinaim 



Turnices 



Archecentre 

 Palamedea 



Ratitae 



Galli 



Evolution of the Intestinal Tract in Aves. 

 (For Coracimorphine and Colymbimorphine read Coraciomorphine and Colymbomorphine.) 



prolonged as in Scolopax. The members of the Coraciomorphine Legion, except the 

 Passeres and a few pseudocentric types, may be derived from a metacentre in which 

 the duodenum tends to widen, and in which the part of Meckel's tract anterior to the 

 diverticulum is produced into two loops. In Plates 21, 22, and 23, and in figures 35, 59, 

 74, and 75, the evolution and affinities of the conformations of the intestinal tract are 

 represented, and by no means necessarily the pedigrees and affinities of the birds 

 containing these conformations. 



INTESTINAL PORTION OF THE ATJTONOMIC NERVOUS SYSTEM. 



It was only at a late stage of this investigation that my attention was directed to the 

 peculiar features in the Sympathetic, or, to use the modern term, the Autonomic Nervous 

 System of Birds. In certain birds (for instance the Megapodidae and Cracidae) the 



