52 THE ANIMAL PARASITES OF MAN 



are capable of developing into the full flagellate on the return of more favourable 

 conditions. These forms are often known as the post-flagellate stage of the 

 organism. When ingested by a new host, the post-flagellate coat becomes more 

 flexible, and the phase of the organism which now recommences growth is known as 

 the pre-flagellate stage ; it gradually develops into the typical flagellate organism. 



Many Flagellata live free in fresh and salt water. They prefer stagnant water, 

 rich in organic products of decomposition, such as puddles, swamps and pools. 

 Those forms developing shells and colonies are, as a rule, adherent. A number of 

 species are parasitic in man and animals, living mostly within the intestine or in the 

 blood. 



It is usual to classify the Flagellata in four orders : Euflagellata, Dinofla;ellata, 

 Choanoftagellata, and Cystoflagellata, of which only the Euflagellata are of interest 

 to us. This is a group comprising numerous species, for the further classification 

 of which the number and position of the flagella are utilised. 



The Euflagellata observed in man belong to the Protomonadina as well as to the 

 Polymastigina. The former possess either only one or two similar flagella, or one 

 principal and one or two accessory flagella. The Polymastigina possess at least 

 three flagella of equal size, or four to eight of unequal size, inserted at different 

 points. An undulating membrane may be present in members of both groups. 



It must also be pointed out that unicellular organisms with one or several 

 flagella are not always classified with flagellates, for such forms occur in Rhizopods 

 as well as temporarily in the lower plants. In addition, the examination of the 

 flagellates, especially the parasitic species, is very difficult on account of their 

 diminutive size and great activity ; thus it happens that certain forms cannot with 

 certainty be included in the group because their description is insufficient. 



Order. Polymastigina, Blochmann. 



The Polymastigina contains flagellates with three to eight flagella. 

 Some of the Flagellata parasitic in man belong to the Polymastigina, 

 and to two or three genera that are easily distinguishable. 



Genus. Trichomonas, Donne, 1837. 



The body is generally pyriform, the anterior part usually rounded, the posterior 

 part pointed. There are at the anterior extremity three (? four) equally long flagella 

 that are sometimes matted together. A blepharoplast (kinetic nucleus) and basal 

 granule are present, together with a supporting structure known as an axial 

 filament or axostyle. In addition there is an undulating membrane, bordered by a 

 trailing flagellum, that commences at the anterior extremity and proceeds obliquely 

 backwards. The nucleus, which is vesicular, is situated near the anterior extremity, 

 and behind it are one or more vacuoles, none of which seems to be contractile. 

 These flagellates are parasitic in vertebrate animals, and live chiefly in the intestine. 



Trichomonas vaginalis, Donne. 



The form of the body is very variable, and is elongate, fusiform 

 or pear-shaped, also amoeboid. The length varies between 15^ and 

 25 ft, and the breadth between 7 //, and 12 p. The posterior extremity 

 is drawn out to a point and is about half the length of the remainder 



