134 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



is aborted, but the crop is swollen out into a simple 

 sac (bees), or into two hemispherical sacs (blowfly), 

 or its attached portion forms a short narrow tube 

 and its free part a swollen bladder-like enlarge- 

 ment (butterfly). This organ may extend far back 

 into the abdomen, and, as it has thin walls and no 

 muscular attachment to the body wall, its size is 

 probably increased and diminished by the contractions 

 of the hinder parts of the body ; this so-called " suck- 

 ing stomach " appears to act as a reservoir for food in 

 the Diptera. 



At the anterior end of the tract there open the 

 ducts of the salivary glands, which are ordinarily 

 developed in insects, but best seen in the haustellate 

 forms ; they vary greatly in form and size, and are 

 by no means always confined to the function of 

 digestive glands, as the mosquito, the bug, or the 

 tse-tse fly are sufficient to bear witness. Many larvae 

 have well-developed glands which open just behind the 

 mouth, and which secrete a body which in air hardens 

 into a fine silky thread. Glands are often developed in 

 the walls of the rectum or large intestine, and have a 

 secretion which is frequently of a pungent, if not of a 

 disagreeable, odour. The Malpighian vessels which 

 are connected with the hinder portion of the tract 

 and open into it are not digestive glands, but organs 

 of renal excretion. (See page 256.) 



We find a very different arrangement of mouth 

 organs in the Mollusca to that which we have just 

 been studying in the Arthropoda ; the great majority 

 are without any seizing organs of any kind, and the 

 lowest, the Lamellibranchiata, have no means by 

 which they can comminute their food ; they live, 

 therefore, on the minute organisms which are brought 

 to them with the water of respiration, and which are 

 felt for and guided to the mouth by the blunt "labial 

 tentacles " that lie on either side of it. 



