Fig. 343.—Urinator lumme, red-throated loon. Small intestine with cæcal pouch; the remnant of the vitello-intestinal duct. (Columbia University Museum, No. 997.)   Fig. 344.—Urinator imber, great northern diver. Small intestine with cæcal pouch; the remnant of the vitello-intestinal duct. (Columbia University Museum, No. 78/1573)

In some birds the small intestine is also provided with a cæcal pouch, the remnant of the vitello-intestinal duct corresponding in its significance to the occasional mammalian diverticulum of Meckel (Figs. 343 and 344). (cf. [p. 35].)

V. ILEO-COLIC JUNCTION, CÆCUM AND VERMIFORM APPENDIX IN THE MAMMALIA.

I. Subclass: Ornithodelphia.

I. Order: Monotremata.

In many particulars the anatomical structure of these animals reveals a close relationship to the Sauropsida. They represent the mammalian class in its lowest stage of evolution.

The ileo-colic junction in all the existing forms is direct, without angular bend at the entrance of the small into the large intestine. The cæcum is a long narrow pouch, slightly dilated at the extremity, derived from the beginning of the colon and extending backward along the free margin of the small intestine. The cæcum resembles in its general shape and structure the pouches seen in many birds, except that it is unilateral, while the birds normally have two symmetrical cæca. The cæcum of Ornithorhynchus anatinus, the platypus or duck bill, is shown in Figs. 345 and 346, and that of Echidna hystrix, the spiny ant-eater, in Fig. 347. These two animals represent the two genera into which the order is divided.

Fig. 345.—Ornithorhynchus anatinus, duck mole.Ileo-colon and cæcum. (Columbia University Museum,No. 1499.)

 

Fig. 346.—Ornithorhynchus anatinus, duck mole. Ileo-colon and cæcum.(Columbia University Museum, No. 1500.)

Fig. 347.—Echidna hystrix, spiny ant-eater. Ileo-colon and cæcum. (Columbia UniversityMuseum, No. 1501.)

II. Subclass: Didelphia.

II. Order: Marsupialia.