Œsophagus and crop ·95 in. Gizzard ·1 Chylific stomach ·5 Small intestine ·1 Colon ·875 Rectum ·25 ----- 2·775 ===== [Illustration: Fig. 56.--Alimentary Canal of Cockroach. × 2.] The principal appendages of the alimentary canal are the salivary glands, the cæcal diverticula of the stomach, and the Malpighian tubules. Considered with respect to its mode of formation, the alimentary canal of all but the very simplest animals falls into three sections--viz., (1) the mesenteron, or primitive digestive cavity, lined by hypoblast; (2) the stomodæum, or mouth-section, lined by epiblast, continuous with that of the external surface; and (3) the proctodæum, or anal section, lined by epiblast folded inwards from the anus, just as the epiblast of the stomodæum is folded in from the mouth. The mesenteron of the Cockroach is very short, as in other Arthropoda, and includes only the chylific stomach with its diverticula. The mouth, œsophagus, and crop form the stomodæum, while the proctodæum begins with the Malpighian tubules, and extends thence to the anus. Both stomodæum and proctodæum have a chitinous lining, which is wanting in the mesenteron. At the time of moult, or a little after, this lining is broken up and passed out of the body. [Illustration: Fig. 57.--Section of Wall of Crop. _Cc_, chitinous layer; _C_, chitinogenous cells; _Mi_, inner muscular layer; _Mo_, outer do. × 275.] The mouth of the Cockroach is enclosed between the labrum in front, and the labium behind, while it is bounded laterally by the mandibles and first pair of maxillæ. The chitinous lining is thrown into many folds, some of which can be obliterated by distension, while others are permanent and filled with solid tissues. The lingua is such a permanent fold, lying like a tongue upon the posterior wall of the cavity and reaching as far as the external opening. The thin chitinous surface of the lingua is hairy, like other parts of the mouth, and stiffened by special chitinous rods or bands. The salivary ducts open by a common orifice on its hinder surface. Above, the mouth leads into a narrow gullet or œsophagus, with longitudinally folded walls, which traverses the nervous ring, and then passes through the occipital foramen to the neck and thorax. Here it gradually dilates into the long and capacious crop, whose large rounded end occupies the fore-part of the abdomen. When empty, or half-empty, the wall of the crop contracts, and is thrown into longitudinal folds, which disappear on distension. Numerous tracheal tubes ramify upon its outer surface, and appear as fine white threads upon a greenish-grey ground. [Illustration: Fig. 58.--Wall of Crop, in successive layers. References as in fig. 57. × 250.]