A second modification of the intestinal canal, suggesting the same physiological interpretation as the ileo-colic caecum, is presented by the so-called pyloric caeca or appendices of many Teleosts and Ganoids already referred to (p. 119). While these structures in some forms very probably have assumed a secretory function (Figs. 476 and 477), they evidently act in others as diverticula in which material undergoing digestion is retained, while they increase at the same time the intestinal mucous secretory and absorbing surface (Figs. 478 and 479). They thus correspond physiologically to the ileo-colic caecum. In this connection it is interesting to note that in Ganoids, which possess both the pyloric appendices and the spiral valve, the two structures develop in inverse ratio to each other, indicating their functional identity. In the serial review of the structure and significance of the vertebrate caecum and ileo-colic junction these functionally allied modifications of other segments of the intestinal canal deserve notice. [Illustration: FIG. 476.--Pyloric caeca of _Gadus callarias_, codfish. (Columbia University Museum, No. 1825.) _A._ Bound together by connective tissue and blood-vessels. _B._ Dissected to show confluence of caeca to form a smaller number of terminal tubes of larger calibre entering the intestine.] [Illustration: FIG. 477.--Alimentary canal of _Accipenser sturio_, sturgeon. Numerous pyloric caeca are bound together to form a gland-like organ. In the smaller upper figure on the left the stomach, mid-gut, and pyloric caeca are seen in section, showing the lumen of the latter and their openings into the mid-gut. The lower left-hand figure shows the mid- and end-gut in section, the latter provided with a spiral mucous valve. (Columbia University Museum, Nos. 1826, 1827, and 1828.)] [Illustration: FIG. 478.--Stomach, duodenum, and pyloric caeca of _Lophius piscatorius_, angler. (Columbia University Museum, No. 1824.)] [Illustration: FIG. 479.--_Pleuronectes maculatus_, window-pane. Stomach and mid-gut with pyloric caeca and hepatic duct. (Columbia University Museum, No. 1432.)] The study of the vertebrate ileo-colic junction proper begins both ontogenetically and phylogenetically with the consideration of the primitive type in which the alimentary tube is not differentiated into successive segments and in which consequently no distinction between mid- and hindgut is found (Fig. 465). An example of this primitive condition is presented by the Cyclostomata, in whom the alimentary canal traverses the coelom cavity as a straight non-differentiated cylindrical tube. Fig. 310 shows the alimentary canal of the Lamprey, _Petromyzon marinus_, and it will be observed that the intestine is provided with the spiral mucous fold above mentioned. From this fundamental type the following main groups are to be derived: I. Symmetrical Form of Ileo-colic Junction. Mid- and Endgut in Direct Linear Continuity. (Fig. 465, I.)