Page:Tropical Diseases.djvu/997

Rh but differs in the lack of an undulating membrane and in the presence of a well-marked cystosome in which there is either a flagellum or a membrane; there are three anterior flagella as in tnchomonas. Suborder 2.—The Octomitidæ, for the most part intestinal parasites, have six or eight flagella, generally arranged in pairs; the body is bilaterally symmetrical in structure. Lamblia intestinalis (Fig. 242) is a common parasite in the human intestine; encystment is a common feature; these cysts are ovoid bodies containing two nuclei. On the ventral surface of the free form there is a sucker-like depression. The general structure of the parasite is sufficiently well shown in Fig. 242.

This class contains four orders of interest:
 * i. Gregarinidea.
 * ii. Eimeriinea.
 * iii. Hæmosporidia.
 * iv. Neosporidia.

This order contains the family of Gregarines. The young parasite (trophozoite) is at first a parasite of the endothelial cells, but later, becoming free, lies in the body cavity and in the intestinal canal. The full-grown parasite is of a large size, and has a definite shape and cuticle. Gregarines are found commonly parasitic in the digestive tract of insects; they are not known to occur in the vertebrata. The anterior end of the body may be provided with a rostrum, termed the epimerite, armed with hooks and processes; the posterior portion of the body may be divided by a single septum into two segments, known as the protomerite and the deutomerite; the nucleus, often of a large size and oval shape, is situated in the latter portion. The complete parasite is capable of independent rapid motion. The Gregarines are divided into two groups, on one of which, Eugregarinæ, a sexual phase— sporogony— only takes place. In the other, Schizogregarines, multiplication is effected by both sporogony and schizogony. The Eugregarines are further divided into two groups, the cephaline and acephaline gregarines. The cephalines have a well-developed epimerite; in the acephalines the body is not segmented. In the sporogonic phase two full-grown gregarines approximate to one another, become rounded, and proceed to secrete a tough envelope. Each gregarine breaks up into a number of gametes which conjugate in pairs. The resulting zygotes (sporoblasts) each encyst in a sporocyst in which eight sporozoites are formed; these on entering a new host develop into adult gregarines. The whole cycle takes place within the same insect or earthworm.