INTESTINAL TEACT OF BIRDS. 213 



relation any of the Falconiformes with Cariama. The latter bird, as vvDl appear later, 

 is definitely a member of the Gruiform assemblage, and for relations between the 

 Gruifonn birds and the Falconiform birds it is necessary, so far as the characters of 

 the intestinal tract take us, to go back to the archecentric type underlying all birds. 



SUMMARY OF THE PELARGO-COLYMBOMORPHINE BRIGADE. (Plate 21.) 



At this point it is convenient to attempt a resume of the conclusions to which study 

 of the Intestinal Tract has so far led. 



Taking the form in Palamedea as the archecentric type, it appears that the Struthious 

 birds are grouped indifferently around it, as they all display the archecentric character 

 in an unmodified or very slightly modified form. Of the Colymbomorphae, Colymbus 

 exhibits the simplest type, in fact a slight but definite modification of the Palamedea 

 form, consisting in the expansion of Meckel's tract into a set of straight, narrow loops, 

 one of which is axial and bears the diverticulum, the others being arranged nearly 

 symmetrically about the middle mesenteric vein. There is usually a supra-duodenal 

 loop ; the caeca are functional and the rectum is short and straight. This form is of 

 great importance, as it is a Pelargo-Colymbomorphine metacentre from which radiate the 

 type of intestinal tract displayed by Gadow's first Brigade of birds, including the Legion 

 Colymbomorphae with the Colymbiformes, the Sphenisciformes, and the Procellariiformes, 

 and the Legion Pelargomorphae with the Ciconiiformes, Anseriformes, and Ealconiform.es. 

 The Grebes are more apocentric modifications of this metacentric type, the change being 

 chiefly a reduction of the number of loops of Meckel's tract with a corresponding increase 

 in the length of the individual loops. A slight but still more apocentric modification of 

 the Pelargo-Colymbomorphine metacentre produces a new central position, which may 

 be called the Steganopod metacentre. The two chief additional characters of this are, 

 firstly, a tendency to complication and lengthening of the duodenum, a tendency which 

 in nearly every case is actually fulfilled; and secondly, the appearance immediately 

 above the caeca of a kink supplied by the rectal vessel, although belonging to the 

 drainage-area of the middle mesenteric vein. Meckel's tract does not differ funda- 

 mentally (fig. 35) from the condition in the Pelargo-Colymbomorphine metacentre, but 

 the caeca are reduced. The Sphenisciformes are modified from such a condition only by 

 the greater length of their gut and consequent increase in the number of minor loops 

 on Meckel's tract. The Procellariiformes are modified from it chiefly in the reduction 

 of the number of loops on Meckel's tract and the great increase in length of the 

 individual loops, the axial of which may be spirally twisted in the most apocentric 

 forms, e. g., the Oceanitidse. Of the Ciconiiformes, the Steganopods retain their meta- 

 centric position. The Ardeae start from that position (Scopus), but in the Ardeidae the 

 minor loops of Meckel's tract become very complicated and one of the two caeca is lost. 

 The Ciconii start from the Steganopod metacentre with forms like Anastomus, but they 

 rapidly reach a more apocentric condition, the chief peculiarity of which is the spiral 

 twisting not only of the duodenum but of the first minor loop of Meckel's tract, and 

 the twisting of these two spirals together, so that sometimes the blood-vessels are in 



SECOND SERIES. ZOOLOGY, VOL. VIII. 33 



