In the smaller upper figure on the left the stomach, mid-gut, and pyloric cæca 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.)

A second modification of the intestinal canal, suggesting the same physiological interpretation as the ileo-colic cæcum, is presented by the so-called pyloric cæca 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 cæcum. 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 cæcum and ileo-colic junction these functionally allied modifications of other segments of the intestinal canal deserve notice.

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 cœlom 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:

Fig. 478.—Stomach, duodenum, and pyloric cæca of Lophius piscatorius, angler. (Columbia University Museum, No. 1824.)   Fig. 479.—Pleuronectes maculatus, window-pane. Stomach and mid-gut with pyloric cæca and hepatic duct. (Columbia University Museum, No. 1432.)

I. Symmetrical Form of Ileo-colic Junction. Mid- and Endgut in Direct Linear Continuity. ([Fig. 465], I.)

1. Ileo-colic junction marked externally by an annular constriction, corresponding to a ring-valve with central circular opening in the interior ([Fig. 465], I, 1).

This form is encountered in many Teleosts. The projecting annular mucous fold resembles the pyloro-duodenal valve.