THE ALIMENTARY CANAL. 53 



spherical organ, formed by the invagination of the posterior 

 extremity of the stomodaeum in the anterior portion of the 

 mesenteron. When it assumes the form of a gizzard, the 

 oesophageal portion is lined by a series of radially symmetrical 

 chitinized folds, which form an efficient organ for the trituration 

 of the food. The form of the proventriculus in the fly is typical 

 of that in all suctorial insects. 



The Mesenteron consists of the chyle stomach and its append- 

 ages, and of that portion of the intestine in front of the 

 Malpighian tubes. Some authors have named the whole 

 of this part of the alimentary canal the chyle stomach. 

 Although there is no distinct line of demarcation between the 

 anterior and posterior portions of the mesenteron in many 

 insects, the two sections of the tube differ considerably, and in 

 some forms are distinctly divided by a pyloric sphincter into a 

 chyle stomach and an intestine. I shall therefore use the term 

 proximal intestine for the posterior portion of the mesenteron. 



The Metenteron begins in front of the orifices of the Mal- 

 pighian vessels, as a distinct thin-walled dilatation of the 

 intestine (PI. I., Fig. i), the lower part of which forms a large 

 diverticulum or caecum in many insects. Beyond this the 

 intestine is much narrowed, and exhibits a thick muscular coat. 

 I shall term it the dhtal intestine, and shall speak of the 

 dilatation as the sinus. This distal intestine is termed the 

 ileum or colon by different writers. 



The rectum or proctodseal involution has a very distinct 

 structure, and is always lined by a thick cuticular intima ; it is 

 usually separated from the metenteron by a valve, which is 

 formed by its invagination within the metenteron ; the valve 

 is not unlike the proventriculus in the disposition of its 

 walls. 



The Glandular Appendages of the alimentary canal are known 

 as the salivary and gastric glands, the Malpighian vessels, and 

 the rectal glands. 



The Salivary Glands. — This term is applied to all those glands 

 in insects which discharge their secretion into the mouth or 

 pharynx. The largest and most constant have a duct, which 



