ENTAMCEBA 65 



chromatic body or ' karyosome ' of the nucleus is centrally 

 placed. Unlike E. coli it may attack the mucous membrane of the 

 intestine, probably by the secretion of an enzyme, so forming 

 ulcers. Some individuals penetrate the blood vessels in the same 

 way, and are carried by the circulation to the liver, where they 

 may set up abscesses. When living free in the gut it feeds on 

 faecal particles just as E. coli does, but when it invades the tissues 

 its chief food is red blood cells ; it can also absorb liquid food. 

 E. histolytica is found all over the world, but only about ten 

 per cent, of those human beings who are infected show clinical 

 symptoms of its presence in the form of dysentery. Its life-cycle 

 (Fig. 37) appears to differ from that of E. coli chiefly in the 

 number of the cyst nuclei, of which there are only four, though 

 after emergence these divide with the cytoplasm to form eight 

 little amoebae. The cysts are the only infective forms, and are 

 acquired from carriers who do not show the disease, either 

 through food or water, or after having been ingested by flies or 

 cockroaches. 



TRYPANOSOMA 



Flagellate Protozoa of the genus Trypanosoma are responsible 

 for various very dangerous diseases of man and animals in warm 

 countries. They are parasitic in the blood and other fluids of 

 backboned animals, but at one stage in the life-history live in 

 invertebrates which suck the blood of the vertebrate hosts. The 

 body (Figs. 38, 39) is worm-like, about one-thousandth of an 

 inch in length, tapering towards the ends, but more pointed in 

 front than behind. The shape of the body is maintained by a 

 strong pellicle. A single flagellum stands at the front end, and 

 from its base an undulating membrane or sheet of contractile 

 protoplasm runs along one side nearly to the hind end. The flag- 

 ellum is continued as a strongly-staining thread along the free 

 edge of the membrane, and terminates behind in a minute ' basal 

 granule ' or blepharoplast, embedded in the cytoplasm. By the 

 working of the undulating membrane and flagellum the animal 

 swims rapidly with a graceful wavy movement, either forwards 

 or backwards. There is no contractile vacuole. Near the middle 

 of the body is an egg-shaped nucleus, and a smaller mass, which 

 stains like the nucleus, stands close to the blepharoplast. It is 

 known as the parabasal body, and was formerly called the kineto- 

 nucleus, but though its functions are unknown it seems to have 



