THE MASTIGOPHORA 169 



accompanied by a definite mitotic division of the nucleus. 

 Alternation of the amoeboid and flagellate forms with reproduction 

 by fission continues for about eighteen days, and then gameto- 

 genesis sets in. The gametes are minute (1-2 p, in length) uni- 

 fiagellate organisms, and soon after their escape they conjugate in 

 pairs to form the zygotes. Gametogenesis occurs in the amoeboid 

 form, without encystment or withdrawal of the pseudopodia, and the 

 number of gametes formed by a single individual may exceed one 

 hundred. After a time the zygotes withdraw their flagella, assume 

 a spherical shape, and then creep into a Volwx individual. 

 Gametogenesis may also occur in the radial form, but it has not 

 been observed in the flagellate form. 



It does not seem to be certain that the species described belongs 

 to the same genus as others that have been attributed to Pseudospora 

 (see p. 8), but the description of its life-history given by Miss 

 Robertson proves that it is not a Proteomyxan, but is correctly 

 placed with the Mastigophora. 



TRIBE 3. POLYMASTIGINA. 



With more than two flagella (exclusive of Multicilia). 



SOB-TRIBE 1. TRIMASTIGINA. Three flagella. Trimastix, Kent ; 

 Dallingeria, Kent ; Elvirea, Paroiia. Costia necatrix, Henneguy, is a 

 flagellate ectoparasite of the trout which cannot live in infusions, but 

 requires very pure water. It is the only flagellate ectoparasite known 

 which cannot live apart from its host. It penetrates into the epidermis 

 of the fry, frequently causing a mortal disease. The adult fishes are 

 immune, being protected from the parasite by their scales. 



SUB-TRIBE 2. MoxosiOMATiNA. 1 Four (rarely six) flagella, one 

 mouth-spot or oral groove, unilateral, asymmetrical. Tetramitus, Perty 

 (Fig. 7 (14)); Collodictyon,C&i'teT; Trichomonas, Donne" ; Trichomastix^loch- 

 mann ; Monocercomonas, Grassi ; Megastoma, Grassi. Megastoma entericum, 

 parasitic in man and domestic animals, is regarded as intermediate 

 between the Tetramitina and the Distomatina (Klebs), having a uni- 

 lateral mouth as in Tetramitus and sextuple flagella as in Hexamitus. 

 Trichomonas, Donne. T. intestinalis is found in the intestine of mice. 

 " It is pear-shaped with three flagella springing from the blunt end, and 

 an undulating membrane with a thickened border passing in a spiral 

 manner round the body and terminating in a free flagellum " (Wenyon). 



SOB-TRIBE 3. DISTOMATINA, Klebs, 1892. Body bilateral but not 

 symmetrical, since the two mouth-spots (oral grooves) are placed on 

 opposite surfaces of the body ; flagella arranged in pairs. Principally 

 found in stagnant water. Gyromonas, Seligo, 4 flagella ; Trigonomonas, 

 Klebs, 6 flagella ; Trepmnonas, Duj., 8 flagella ; Hexamitus, Duj. (Fig. 7 

 (5)), and Urophagus, Klebs, with 6 or 8 flagella, of which two or three 

 pairs are anterior and the fourth pair are gubernacula (Schleppgeisseln). 

 The two last-named genera are, alone among Flagellata, characterised by 



1 Including the Tetramitina, with a wider significance. 



