The stomach of a flea is a pear-shaped sack which occupies an appreciable part of the insect’s abdomen. That it is capable of containing a comparatively large amount of blood is apparent from the observation that after a flea has enjoyed a good meal nearly the whole of the abdomen is seen to be filled with a bright red mass. During the investigation of the part played by fleas in spreading plague an endeavour was made to measure, as accurately as possible, the average capacity of a rat-flea’s stomach when filled with blood. Healthy fleas, taken from Bombay rats, were starved for twelve hours, and at the end of that time were fed on healthy animals. The stomach was then dissected out whole and floated in a salt solution. Any adherent organs or muscles were carefully removed. Under these conditions the stomach can be examined and measured under the microscope. The average capacity of a rat-flea’s stomach has been approximately estimated to be half a cubic millimetre. The stomach of a flea is therefore, comparatively speaking, very large. The blood remains in the stomach in a partially digested condition. It gradually diminishes in volume, showing clearly that absorption is taking place. At the end of so much of the digestive process as takes place in the flea’s stomach, the blood has become reduced to a thick, slimy, dark red mass. This passes down the intestine to the rectum, where it is perhaps further influenced by the secretion of the so-called rectal glands. Finally, the undigested remains pass from the rectum in the form of very minute, round, almost black, tarry drops. The terminal section of the flea’s digestive canal is called the rectum. Here are placed the rectal glands (Fig. 6), which are six in number. Their function seems not to be certainly known. The external opening of the rectum is placed at the extreme end of the flea’s body between the tergite and sternite of the tenth segment.