The gizzard has externally the form of a blunt cone, attached by its base to the hinder end of the crop, and produced at the other end into a narrow tube (1/4 to 1/3 in. long), which projects into the chylific stomach. Its muscular wall is thick, and consists of many layers of annular fibres, while the internal cavity is nearly closed by radiating folds of the chitinous lining. Six of the principal folds, the so-called “teeth,” are much stronger than the rest, and project so far inwards that they nearly meet. They vary in form, but are generally triangular in cross section and irregularly quadrilateral in side view. Between each pair are three much less prominent folds, and between these again are slight risings of the chitinous lining. A ridge runs along each side of the base of each principal tooth, and the minor folds, as well as part of the principal teeth, are covered with fine hairs. The central one of each set of secondary folds is produced behind into a spoon-shaped process, which extends considerably beyond the rest, and gradually subsides till it hardly projects from the internal surface of the gizzard. Behind each large tooth (_i.e._, towards the chylific stomach) is a rounded cushion set closely with hairs, and between and beyond these are hairy ridges. (See fig. 61.) The whole forms an elaborate machine for squeezing and straining the food, and recalls the gastric mill and pyloric strainer of the Crayfish. The powerful annular muscles approximate the teeth and folds, closing the passage, while small longitudinal muscles, which can be traced from the chitinous teeth to the cushions, appear to retract these last, and open a passage for the food.[123] [123] Plateau has expressed a strong opinion that neither in the stomach of Crustacea nor in the gizzard of Insects have the so-called teeth any masticatory character. He compares them to the psalterium of a Ruminant, and considers them strainers and not dividers of the food. His views, as stated by himself, will be found on p. 131. [Illustration: Fig. 60.--The Six Primary Folds (teeth) of the Gizzard, seen in profile.] [Illustration: Fig. 61.--Part of Gizzard laid open, showing two teeth (_T_) and the intermediate folds, as well as the hairy pads below. _A-A_ and _B-B_ are lines of section (see figs. 62 and 63). × 50.] [Illustration: Fig. 62.--Section through one tooth and two intermediate spaces (see figure 61, _A-A_). _Cc_, chitinous cuticle; _C_, chitinogenous layer; _am_, annular muscles; _p_, peritoneal layer. × 75.] [Illustration: Fig. 63.--Section through one principal hairy ridge and two intermediate spaces (see fig. 61, _B-B_); _rm_, radiating muscles; _tr_, trachea. The other references as before. × 75.]