354 THE ANATOMY OF INVERTEBRATED ANIMALS. 



each side of the oesophagus and crop, extending through the 

 thorax, as far as the commencement of the abdomen. Each 

 gland is a white mass, as much as a quarter of an inch long, 

 and composed of numerous acini. The ducts which arise from 

 these acini unite first into a single trunk on each side, and 

 then, beneath the sub-cesophageal ganglion, the two trunks 

 join to form the single short salivary duct which opens be- 

 neath the lingua. The ducts of the salivary glands are lined 

 by a transversely-ribbed chitinous membrane, so that they 

 greatly resemble tracheae. 



The salivary receptacles (Fig. 98, k) are elongated oval 

 sacs, three-eighths of an inch long, each of which is situated 

 at the extremity of a long duct. The ducts unite in front with 

 one another, and with the duct of the gland, to form the short 

 terminal common duct. The receptacle and its ducts have a 

 chitinous lining similar to that of the duct of the glands, but 

 the spiral marking does not extend over the walls of the re- 

 ceptacle. 



The proventriculus has a thick muscular coat, and the chi- 

 tinous lining which is continued into it from the ingluvies is 

 greatly thickened, and produced into six hard, brown, ridge- 

 like principal teeth. Posterior to these is a circle of six 

 prominent cushions covered with setae, and similar setae beset 

 the lining membrane of the funnel-shaped cavity into which 

 they project. Between each pair of principal teeth are five 

 smaller tooth-like ridges, of which the median is the largest, 

 and a variable number of still finer longitudinal elevations lie 

 between them. 



The proventriculus leads posteriorly into a narrow, thick- 

 coated canal, the tubular extremity of which projects freely 

 into the much wider anterior end of the chylific ventricle, and 

 constitutes a very efficient valve. 



The short and narrow anterior division of the intestine 

 (ileum) is separated from the colon by a circular valve, the 

 surface of which is beset with small spines. 



The Malpighian glands are very numerous (20-30), deli- 

 cate, caecal tubules, of even diameter throughout, and lined 

 by a small-celled epithelium inclosing a central cavity. 



The communication between the colon and the rectum is 

 very narrow, but is not valvular. The walls of the rectum 

 itself are raised into six ridges, which project into its inte- 

 rior and are abundantly supplied with tracheae ; these are the 

 so-called rectal glands. Anal glands appear to be absent. 



The histology of the alimentary canal has been particu- 



