ECOLOGY 27 



toxic substances produced by parasitic Protozoa is meager. 

 Sarcosporidia appear to produce a certain toxic substance which, 

 when injected in the blood vessel, is highly toxic to experimental 

 animals. This was named sarcocystine (Laveran and Mesnil) or 

 sarcosporidiotoxin (Teichmann and Braun). As in bacterial in- 

 fection, the reaction and resistance of the host to protozoan in- 

 fection apparently differ among different individuals. Taliaferro 

 demonstrated that there occur in the blood of animals suffering 

 from trypanosomiasis or malaria, certain agents which would 

 either inhibit the rate of multiplication of the parasites or destroy 

 the parasites themselves. 



With regard to the origin of parasitic Protozoa, it is generally 

 agreed among biologists that the parasite in general evolved from 

 the free-living form. The protozoan association with other organ- 

 isms was begun when various protozoans which lived attached 

 to, or by crawling on, submerged objects happened to transfer 

 themselves to various invertebrates which occur in the same 

 water. These Protozoa benefit by change in location as the host 

 animal moves about, and thus enlarging the opportunity to ob- 

 tain a continued supply of food material. Examples of such 

 ectocommensals abound everywhere. The ectocommensalism may 

 next lead into ectoparasitism as in the case of Costia or Hydra- 

 moeba, and then again instead of confining themselves to the 

 body surface, the Protozoa may bore into the body wall from out- 

 side and actually acquire the habit of feeding on tissue cells of 

 the attached animals as in the case of Ichthyophthirius. 



The next step in the evolution of parasitism must have been 

 reached when Protozoa, accidentally or passively, were taken 

 into the digestive system of the Metazoa. Such a sudden change 

 in habitat appears to be fatal to most protozoans. But certain 

 others possess extraordinary capacity to adapt themselves to an 

 entirely different environment. For example, Dobell (1918) ob- 

 served in the tad-pole gut, a typical free-living limax amoeba, 

 with characteristic nucleus, contractile vacuoles, etc., which was 

 found in numbers in the water containing the fecal matter of the 

 tadpole. Glaucoma pyriformis (p. 548), a free-living ciliate, was 

 found to occur in the body cavity of the larvae of Theohaldia 

 annulata (after MacArthur) and in the larvae of Chironomus 

 plumosus (after Treillard and Lwoff). Lwoff successfully inocu- 

 lated this ciliate into the larvae of Galleria niellonella which died 



