iv] MODES OF NUTRITION 105 



all the parasite has to do is to recombine the food- 

 stuffs into the specific proteids, fats, and carbohy- 

 drates of its own tissues. The process of digestion is 

 abbreviated in the case of those parasites which live 

 on the external surfaces of their hosts, for there the 

 food consists of mucus which is probably easier to 

 digest than the proteid of a captured animal. In the 

 most profound parasitic conditions, as when such a 

 form as a trypanosome inhabits the blood-stream or 

 the cerebro-spinal fluid; or when a blood-sucking 

 parasite lives on the external surface of its host with 

 its suctorial mouth embedded in one of the larger 

 blood-vessels, it simply absorbs the ready-made 

 proteids of the host, possibly in the same way as the 

 cells of the latter also absorb them. Essentially then 

 the parasitic mode of nutrition is one in which the 

 digestion of food-stuff is performed outside the body 

 of the organism and by the agency of some other 

 organism. It may be called the saprophytic mode of 

 nutrition when we speak of a plant parasite, and the 

 saprozoic mode when we speak of an animal parasite. 

 Now let us consider parasitism in the open that 

 is a parasitic mode of life practised by an animal 

 which does not live in the cavities, or on the body of 

 another animal. It seems paradoxical so to speak, 

 but we must remember our definition of the habit 

 that it consists in essence of the utilisation of already 

 digested food ; and further that it is possible to rear 



