Three layers can be distinguished in the wall of the crop--viz., (1) the muscular, (2) the epithelial, and (3) the chitinous layer.[122] The muscular layer consists of annular and longitudinal fibres, crossing at right angles. (See fig. 58.) In most animals the muscles of organic life, subservient to nutrition and reproduction, are very largely composed of plain or unstriped fibres. In Arthropoda (with the exception of the anomalous Peripatus) this is not generally the case, and the muscular fibres of the alimentary canal belong to the striped variety. The epithelium rests upon a thin structureless basement-membrane, which is firmly united in the œsophagus and crop to the muscular layer and the epithelium. The epithelium consists of scattered nucleated cells, rounded or oval. These epithelial cells, homologues of the chitinogenous cells of the integument, secrete the transparent and structureless chitinous lining. Hairs (setæ) of elongate, conical form, and often articulated at the base, like the large setæ of the outer skin, are abundant. In the œsophagus they are very long, and grouped in bundles along sinuous transverse lines. In the crop the hairs become shorter, and the sinuous lines run into a polygonal network. The points of the hairs are directed backwards, and they no doubt serve to guide the flow of saliva towards the crop. [122] Here, as generally in the digestive tube of the adult Cockroach, the peritoneal layer is inconspicuous or wanting. It occasionally becomes visible--_e.g._, in the outer wall of the Malpighian tubules, and in the tubular prolongation of the gizzard. [Illustration: Fig. 59.--Transverse section of Gizzard of Cockroach. The chitinous folds are represented here as symmetrical. See next figure. × 30.]