INTESTINAL TRACT OF ETRDS. 273 



zoological characters. "When the archecentricities and apocentricities of a number of 

 organs or structures liavc been mapped out, the questions ai*e opened as to bow far 

 arcbecentric conditions of different organs are combined in the same animal ; how far 

 apocentric conditions ai'e so combined ; how far arcbecentric conditions of one set of 

 structures are associated witli apocentric conditions of other sets. In general terms, the 

 problem is how far different organs become modified or remain unmodified independently 

 or in association in tbc same individuals. If we compare the l)og-fish and the Frog, we 

 find that, in relation to the general characters of the Iclitbyopsida, the Dog-fish is arche- 

 centric as regards its skeletal system, apocentric as regards its urinc-genilal system, 

 while tbc Frog is apocentric in its skeleton and arcbecentric in its ui"i no-genital system. 

 Naturally, in a ])a[)er dealing with part of one system, this problem of association has 

 not been raised, but I desire to add my conviction, based on a general study of birds and 

 a particular study of small groups of birds, that the larger the number of forms in a 

 ^roup and the nari-ower the anatomical range within which the group is confined, the 

 more close is tlie association of apocentric modification of different organs. 



BIBLIOGRAPHY. 



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 Zool. Jaliib. vol. V. pp. 181-210. 



(2) Beduard, F. E. — The Structure and Classificatiou of Birds. London, 1898. 



(3) Berrv, R. J. A. — Tlie True Csecal Apex or the Vermiform Appendix ; its Minute and Comparative 



Anatomy. Journ. of Anat. xxxv. pp. 83-100. 



(4) Birmingh.\m, a. — Some Points in the Anatomy of the Digestive System. Journ. of Anat. xxxv. 



pp. ;5;5-6(i. 



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(7) Evans, .\. II.— Birds. London, 1899. 



(8) Finn, F. — A Guide to the Zoological Collections in the Bird (Jallcry of the Indian ^luscuni. 



Calcutta, 19tK). 



(6) FijRHKixoER, M. — Untcrsuchungcn znr Morphologic und SystematiU der \'ogcl. 1888. 



(ic) Gadow, H. — \ ersuch einer verglcichenden Anatomic des Verdauungssystemcs der Vogel. Jenaische 



Zeitschr. xiii. 1879, pp. 92-171 & 339-103. 

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pp. 303-315. 



(12) Id. — "Vogel " in Bronn's Klassen und Ordnungcn des Thicr-Reich. I89I. 



(13) Garrod, .\. H. — On the Anatomy of the Hoatzm. Froc. Zool. Soc. 1879, p. 109. 



(14) Gegenuauk, C. — Verglcichcnde Anatomic der Wirbclthicre. 18S9. 



(15) Home, Sir E. — On the ditlerent structures and situations of the Solvent (jlands of Birds 



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 (16) Id. — The course of the lutcsliue, with the Varieties ic the form of tiie Cieca in the Carni\ orous, 

 Piscivorous, aud Graminiferous Birds. Phil. Trans. 1814. 



