MASTIGOPHORA 51 



backwards, which occur in the Heteromastigoda and are used for clinging, are 

 termed trailing flagella or tractella. At the base of the flagellum, which is almost 

 always at the anterior end, a Choanoflagellate possesses a cytoplasmic funnel-shaped 

 neck or collar. In the parasitic forms an undulating membrane is often present. 



The body of the Flagellata is usually small, generally elongate and of un- 

 changeable form. It is frequently covered by a distinct cuticle, and, in certain groups, 

 by a hard envelope, or it may be more or less loosely enveloped by a gelatinous or 

 membranous covering. An ectoplasmic layer is thin and not always obvious. The 

 granular cytoplasm contains a varying number of vacuoles, one of which may be 

 contractile, and is generally situated near the area from which the flagella arise, 

 that is, at the anterior extremity. The cytoplasm, moreover, contains the nucleus, 

 which is nearly always single ; and in many species there are also yellow, brown, 

 or green chromatophores of various shapes, such as occur in plants. Some 

 species feed after the manner of green plants (holophytic), or of plants devoid of 

 chlorophyll (saprophytic) ; others, again, ingest solid food, and for this purpose 

 usually possess a cytostome ; the latter, however, in a few forms is not used for its 

 original function, but is connected with the contractile vacuole. Many parasitic 

 forms feed by endosmosis. A few species possess eye-spots with or without light- 

 refracting bodies. 



Variation in the form of the nuclear apparatus occurs. One nucleus only, which 

 may be compact or vesicular, is known in many species. This nucleus is situated 

 either centrally or sometimes near the flagellar end of the body, but its position is 

 subject to variation. The flagella may arise near the nucleus. Other structures, 

 such as an axial filament and a rhizoplast, may be present. Some flagellates are 

 binucleate, the two nuclei which often differ in size and shape being separated 

 from each other. One of these nuclei is the principal, vegetative or trophic nucleus ; 

 the other is an accessory nucleus, frequently termed the blepharoplast, flagellar 

 or kinetic nucleus. One or more small basal granules are often present at or very 

 near the origin of the flagella. 



Multiplication is by fission, usually longitudinal, which may occur in either the 

 free or encysted forms. Division is initiated by that of the nucleus or nuclei 

 (especially the kinetic nucleus). The basal granule divides also. Collars and 

 chromatophores, if present, likewise separate into two. Variation in the method of 

 doubling the original number of flagella occurs. In most organisms, especially 

 uniflagellate forms, the flagellum splits lengthwise, after division of the basal granule, 

 blepharoplast and nucleus. The daughter flagella may be of the same or different 

 lengths and thicknesses. Other flagellates at division are said to produce new 

 flagella in the neighbourhood of the original ones. The daughter organisms in such 

 cases are provided with one or more parental flagella in addition to newly formed 

 ones. It has been stated that in certain cases the parent flagellate retains all its 

 flagella, while new ones arise ab initio in the cytoplasm of the daughter forms. 



Multiplication by longitudinal fission may be interrupted sooner or later by the 

 production of gametes, which form zygotes, from which new generations of indi- 

 viduals arise. In many flagellates gamete formation and sporogony are unknown, 

 and asexual reproduction by fission alone prevails. 



Incomplete division results in the formation of colonies of individuals. These 

 colonies must not be confused with the aggregation rosettes of flagellates found 

 among the parasitic Mastigophora. The individuals of aggregation rosettes are 

 capable of immediate separation from the rosette at will. 



A number of parasitic Flagellata produce non-flagellate stages which are very 

 resistant to external conditions, the assumption of which forms serves to protect the 

 organisms during their transference from one host to another. Such non-flagellate 

 forms possess one or more nuclei, are usually of an oval or rounded contour, and 



