42 THE ANIMAL PARASITES OF MAN 



flagellum is called the principal flagellum ; the smaller ones on the same 

 creature are called accessory flagella. The flagella directed backwards, which 

 occur in the Heteromastigodes and are used for clinging, are termed trailing 

 flagella. At the basis of the flagella, which are almost always at the 

 anterior end, the Choanoflagellata possess a plasmatic funnel-shaped neck. 

 In the parasitic forms an undulating membrane is frequently present. 



The body of the Flagellata is usually small, generally elongated, and of 

 unchangeable form ; it is frequently covered by a distinct cuticle, and, in 

 certain groups, by a solid armature, or it may be more or less loosely envel- 

 oped by a gelatinous or membranous covering. An ectosarc layer is not usually 

 observable. The granular plasm contains a varying number of vacuoles, 

 one of which in most cases is always contractile, and is generally situated 

 at the place from which the flagella arise, that is, at the anterior extremity. 

 The plasm, moreover, contains the nucleus, which is nearly always single ; 

 and in many species there are, besides, yellow, brown, or green chromatophores 

 of various shapes, such as occur in plants. Some species feed after the manner 

 of green (holophytic) plants, or of plants free from chlorophyll (saprophytic) ; 

 others, again, ingest solid food, and for this purpose usually possess a cyto- 

 stom ; the latter, however, in a few forms is not used for its original function, 

 but is connected with the contractile vacuole. A few species, such as 

 Trichocysts, present eye-spots with or without light-refracting bodies. 



Propagation takes place by means of division, mostly longitudinal division, 

 and after previous division of the nucleus, eventually of the neck and the 

 chromatophores. The flagella during this process appear to be regularly cast, 

 so that each daughter-individual must develop fresh flagella, as is the case 

 with the cytostom. Division may take place in the free motile condition, 

 or, in the encysted state, eventually within the capsule. Divisions imper- 

 fectly accomplished give rise, to colonies. Copulation from time to time 

 amongst individuals of the same or different form is very general amongst 

 the flagellata. The product of copulation, the zygote, becomes parent to the 

 free motile generation by means of successive divisions ; this affords an 

 alternation of generations, as the latter multiply without previous copulation. 



Most Flagellata live free in fresh and salt water ; they like stagnant water, 

 and are found in waters 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 into four orders : Euflagellata t 

 Dino flagellata, Choanoflagellata, and Cysto flagellata, 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 closely approach to the Protomonadines 

 as well as to the Polymastigines ; the former possess either only one or two 

 similar flagella, or one principal and one or two accessory flagella, whereas 

 the Polymastigines possess at least three flagella of equal size, or four to 

 eight of unequal size, inserted at different points. An undulating mem- 

 brane may be present in members of both groups ; during rapid motion it 

 conveys the impression of a row of cilia and has often been mistaken for 

 the latter. 



