DIGESTION AND ABSORPTION 663 



is well advanced. In both cases the cells have a centrally placed nucleus 

 and contain spherical globules, yellow in the fresh condition and intense- 

 ly black when stained with haematoxylin. 



The wall of the rectum is very thin, and is lined by a single layer 

 of flattened cells ; these never show much variation in shape. The 

 Malpighian tubes are lined by a single layer of large columnar or cubical 

 cells, which become flattened out in the distended state of the organs. 

 Their nuclei are very large and of a vesicular nature, and the protoplasm 

 has a striated appearance. The lumen of the tubes is occupied by 

 masses of oval or dumb-bell-shaped crystals, believed to be of the nature of 

 uric acid. 



The process of digestion has been studied by Samson in Ixodes ricinus 

 and by Christophers in Ornithodonis savignyi. As the subject is of 

 considerable importance, ticks being the invertebrate 



hosts of the Piroplasmata, which have an intracellular ^e process of diges- 

 tion 



phase, their observations are here summarized. 



On sectioning a piece of the mid-intestine or one of the diverticula 

 for the structure is the same in both of a female just after metamor- 

 phosis and before impregnation, the black globules referred to above, 

 which were produced during the digestion of blood in the nymphal 

 stage, are seen to be present only in small numbers. The gut is sunk 

 inwards and the basement membrane has a folded and pleated appear- 

 ance. The cells are mostly small and contain many vacuoles ; the 

 nucleus is clear and has very little chromatin in it, so that the nucleolus 

 is visible. In specimens which have been allowed to attach themselves 

 to a host and to commence to suck blood, and removed after a short time, 

 the lining cells are seen to be flattened out by the blood and to form a 

 thin layer ; if the tick is allowed to become almost replete with blood 

 and then removed and a piece of the intestine sectioned, it will be seen 

 that digestion has already commenced. The lining cells become elon- 

 gated, project into the blood and become full of vacuoles and plasma 

 fibrils. The blood soon becomes altered into a dark, red, sticky fluid, 

 all its cellular structures becoming broken down and unrecognizable ; at 

 this stage digestion is completed and absorption begins. 



As soon as the blood is broken up, and this takes place with great 

 rapidity, the intestinal cells increase enormously in number and in size, 

 and project into the lumen of the gut ; between the large cells there are 

 many smaller ones which take the place of the former as they become 

 extruded. The large cells become filled with plasma fibrils, and their 

 nuclei are pushed forwards so that they come to lie towards the 



