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928 of Viereck. The process of cyst formation by a budding process of the vegetative form, as described by Schaudinn, is now held to be incorrect. The vegetative forms of the pathogenic and non -pathogenic species are further distinguished by differences in their protoplasmic contents. In E. histolytica the endoplasmic zone is granular, the ectoplasmic hyaline, while in E. coli no such differentiation occurs. The nuclei of the two forms have also distinctive differences. E. coli has a large and distinte nucleolus (karyosome) and a distinct nuclear membrane; whereas the E. histolytica nucleus is not so markedly differentiated (see p. 514). Amœba limax and A. terricola are free-living forms of amœbæ occurring in water and earth. Their cystic stages possess a hard and refractile cyst wall. They are of importance to the tropical pathologist in that they may be ingested and recovered from the stools of dysenteries and others in culture on Musgrave and Clegg's medium. The parasitic amœbæ proper to man have not been so cultivated.

The Mastigophora are divided into three subclasses, of which only one subclass, that of the Flagellata, containing the more typical forms, deserves our attention. The subclass Flagellata is divided into four orders:
 * i. Pantasomina.
 * ii. Protomonadina.
 * iii. Polymastigina.
 * iv. Euglenoidina.

The second and third only of these orders contain parasitic genera.

The Protomonadina are mostly flagellates of a small size with a principal and one or more subsidiary flagella. They are either saprophytic or parasitic. Suborder 1.— Cercpmonas is a frequent parasite in the intestine of man; it is oval in shape, with a single nucleus and well-marked nucleolus, and has two flagella, one passing forwards, and the other backwards over the surface of the body to the hinder end, which is frequently drawn out into a protrusion resembling a tail. Suborder 2.— Bodo (Fig. 225) is similar in shape and general structure to Cercomonas, but has two flagella, one directed forwards, the other backwards as a trailing flagellum; it occurs both as free-living forms and as parasitic in animals, for the most part in the digestive tract. Suborder 3.— Prowazekia.— Prowazekia resembles Bodo in the arrangement of its flagella, but in the possession of a tropho- and a kinetonucleus it more resembles Trypanoplasma. It differs from Trypanoplasma in that the backward-directed