INFECTION OF HAEMATOCOELE 735 



Exit of the parasite per anum is probably a commoner and is cer- 

 tainly a more easily understood event. Infection in this case is depend- 

 ent on the well known habit of defaecation at the 



rrj- j-,i r , , Parasites in the Faeces 



time of feeding, or immediately after it, which is to 



be observed in the great majority of blood-sucking insects. The mechan- 

 ism of the alimentary canal is apparently a very simple one, the mere 

 entry of food into the anterior parts being sufficient to set up a peri- 

 stalsis along the whole length of the gut, with the result that the contents 

 of the more posterior parts are passed out on to the skin of the host 

 while the meal is still in progress. In many cases, and especially 

 among the Diptera, a drop of red and apparently unaltered blood 

 is passed out towards the end of the meal. The excrement thus voided 

 is of course in close approximation to the wound made by the mouth 

 parts, and may mingle with the drop of blood which oozes out when 

 the proboscis is withdrawn, so that there is ample opportunity for 

 the parasite, especially if it is motile, to pass into the tissues of 

 the invertebrate through the wound. This is, in all probability, the 

 means by which the spirochaete of relapsing fever passes from 

 Ornithodorus moubata to the vertebrate host, as has been shown by the 

 experiments of Leishman and Hindle, though in this case it should be 

 noted that the infective fluid is probably the secretion from the Mal- 

 pighian tubes. Exit per anum is also the common event in the natural 

 flagellates of insects, which are either voided on the food of the adult 

 and thereafter taken up by another individual, or pass out in such 

 situations that they may be taken up by the larva with its food. 



Many parasites do not remain free in the lumen of the gut. They 

 may, as in the case of Trypanosoma lewisi described by Minchin, become 

 intracellular at one stage of the developmental cycle, and again become 

 free. In other cases, of which the parasite of malaria offers a good 

 example, they pass into the wall of the gut and remain there for a time. 

 The spirochaete of African Relapsing fever, according to Leishman, 

 undergoes a part of its development in the cells of the Malpighian tubes 

 of the tick, eventually leaving through the anus as indicated above. 



In the more complex cases the parasite leaves the gut altogether by 

 penetrating the wall, and passes into the haematocoele. Subsequent 



events depend on conditions of which we have little 



11 I,- r 1 i i . 11 Passage of Parasite 



knowledge, and this stage of the development is usually to Haematoco | e 



the one which has been the least clearly defined. It is 



clear, however, that one of three things must happen ; the parasite (1) may 



remain in the cavity, (2) pass back to the gut at a later stage, or (3) pass 



