116 THE COCKROACH: 



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 trachea! tubes ramify upon its outer surface, and 

 appear as fine white threads upon a greenish-grey ground. 



Three layers can be distinguished in the wall of the crop- 

 viz., (1) the muscular, (2) the epithelial, and (3) the chitinous 

 layer.* The muscular layer consists of annular and longi- 

 tudinal 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 



**/:, -;7V 



Cc- 



Fig. 58. Wall of Crop, in successive layers. References as in fig. 57. X 250. 



epithelium rests upon a thin structureless basement-membrane, 

 which is firmly united in the oesophagus 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 (seta)) of elongate, conical form, and often articulated at 

 the base, like the large setas of the outer skin, are abundant. 



* 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. 



