As a rule carnivorous birds have short and rudimentary pouches (Figs. 333 and 334), whereas they are long in herbivorous forms (Figs. 335 and 336). Some carnivorous birds, as _Corvus_, _Strix_, etc., have fairly long caeca (Fig. 337). In the passerine birds living on seeds and insects, the caeca are of considerable length as they are also in some of the piscivorous divers (Figs. 338 and 339). They are long in the Ratitae, and in the Lamellirostra, who feed chiefly on plants (Fig. 340). [Illustration: FIG. 337.--_Bubo virginianus_, great horned owl. Ileo-colic junction and caeca. (Columbia University Museum, No. 672.)] [Illustration: FIG. 338.--_Urinator lumme_, red-throated loon. Ileo-colic junction and caeca. (Columbia University Museum, No. 1001.)] [Illustration: FIG. 339.--_Merganser serrator_, red-breasted merganser. Ileo-colic junction and caeca. (Columbia University Museum, No. 1798.)] [Illustration: FIG. 340.--_Casuarius casuarius_, cassowary. (Columbia University Museum, No. 1799.)] [Illustration: FIG. 341.--_Struthio africanus_, African ostrich. Ileo-colic junction and caeca. (Columbia University Museum, No. 48/1573.)] The enormously elongated caeca of the African ostrich contain a spiral fold of the mucous membrane in the interior (Fig. 341). In place of the usual double avian caecum a single pouch is found in a few forms, namely in the Herons (Fig. 342). [Illustration: FIG. 342.--_Ardea virescens_, green heron. Ileo-colic junction and caecum. (Columbia University Museum, No. 1132 a.)] [Illustration: FIG. 343.--_Urinator lumme_, red-throated loon. Small intestine with caecal pouch; the remnant of the vitello-intestinal duct. (Columbia University Museum, No. 997.)] [Illustration: FIG. 344.--_Urinator imber_, great northern diver. Small intestine with caecal 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 caecal 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, CAECUM 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.