Page:Tropical Diseases.djvu/992

934 In this situation the parasite assumes the shape and appearance of a leishmania.

The developmental cycle of this trypanosome takes place in a bug, Lamus megistus, and is shortly as follows: The trypanosomes taken up into the bug's stomach change in about six hours; having lost their flagellum they assume a leishmania form and multiply by fission. After a while these forms assume a crithidial shape and pass into the intestine, eventually gaining the body cavity and salivary glands of the insect as small trypanosomes, and in this manner pass into the vertebrate host.

The changes undergone by trypanosomes in culture on N.N.N. medium (p. 158) are diverse and striking; they then exhibit a series of forms quite distinct from those seen in the blood of the vertebrate, but bearing more resemblance to those seen in the invertebrate host. In such cultures they assume crithidial, leptomonas, and even leishmania forms.

2. (Fig. 235).—The species of this genus parasitic in blood are only known as yet to occur in fresh-water fishes, and are transmitted by leeches. A number of species are parasitic in the organs of snails, such as T. helicis, found in the receptaculum seminis of Helix pomatia. The principal feature is the presence of two flagella arising together from two blepharoplasts at the anterior extremity. One flagellum projects forwards, passing down the side of the body to the hind end, forming the free edge of the undulating membrane and projecting freely backwards as a free flagellum.



3. (Fig. 236).—The distinctive feature of this genus is the relatively short undulating membrane and flagellum arising from the centre of the body at a point in close proximity to the kinetonucleus. The shape of the body varies considerably. Crithidial stages are commonly found as developmental forms of trypanosomes. There remain, however, a number of forms which are parasitic in the intestinal tract of insects, notably the Chironomidæ and Pulicidæ.

4. .—Intestinal parasites of insects, especially muscidæ and hemiptera. In