THE MASTIGOPHORA 259 



Structure. The body-form is of three principal types : (1) An 

 envelope or tough cortex may be entirely absent, and the body is 

 then amoeboid, as in the Rhizomastigina (Figs. 38, 40) ; (2) a thin 

 cuticle may be present, insufficiently rigid to inhibit changes of 

 body-form due to contractility of the living substance (Fig. 15) ; 

 (3) a thicker cuticle necessitates a constant body-form, which is 

 either rigid and unalterable or sinuous and permitting movements 

 of flexion and torsion. In the second type are comprised forms 

 termed commonly " metabolic," on account of the changes of form 

 they exhibit ; contractions of the superficial layer of the body pass, 

 as it were, in waves from the anterior to the posterior end of the 

 body, in a manner similar to the peristaltic contractions of the 

 intestine, producing rhythmic form-changes in the body. 



In species in which the cuticle is thin or absent, a constant body- 

 form may nevertheless be maintained by internal form-giving 

 organs, such as the axostyle of Trichomonas (Fig. 5), Lophomonas 

 (Fig. 45), etc. True internal skeletons, however, do not occur. An 

 external shell or house may be present, enclosing the whole body. 



The protoplasmic body shows, in the amoeboid forms such as 

 the Rhizomastigina (p. 268), distinct ectoplasm and endoplasm. 

 But as a general rule the thin ectoplasm is converted into a firm 

 cuticle, or periplast, enclosing the body and containing contractile 

 elements myonemes. Hence the ectoplasm appears at first sight 

 to be absent, and the protoplasmic body to consist of endoplasm 

 alone. In larger forms the myonemes can be made visible by 

 suitable treatment (Fig. 28), but as a general rule in such minute 

 organisms the existence of myonemes or other contractile mechan- 

 isms can only be inferred from the movements of contractility or 

 flexibility which the body exhibits. 



The flagella may perform various functions in different cases ; 

 they may serve as organs of locomotion and of food-capture, as 

 organs of temporary attachment, and as tactile organs. As stated 

 above (p. 52), they may be distinguished by their relation to the 

 progression of the organism, as tractella, anterior, and pulsella, 

 posterior in movement. The flagella vary in number and in arrange- 

 ment in different species, and for the different types of the flagellar 

 apparatus a number of technical terms are in use : monomastigote, 

 with a single flagellum (Fig. 38) ; isomastigote, with two or four 

 flagella of equal length (Fig. 43) ; paramastigote, with one long 

 principal flagellum and a short accessory flagellum (Fig. 15) ; 

 heteromastigote, with one or more anterior flagella directed forwards, 

 and a " trailing flagellum " directed backwards (Figs. 5, 25) ; 

 polymastigote, with a tuft of flagella (Fig. 45) ; and holomastigote, 

 with numerous flagella scattered evenly over the body (Fig. 113). 

 Of these various types of arrangement, the heteromastigote con- 



