THE ORGANIZATION OF THE PROTOZOA 



53 



cv. 



movements,* which drag the body along in a succession of more or 

 less distinct jerks. 



In many flagellated organisms, forwardly-directed flagella may 

 be combined with so-called "trailing flagella" (" Schlepp-geissel "), 

 which are directed backwards, running along the side of the body, 

 either quite free (Fig. 25) or united to the body 

 by an undulating membrane (Fig. 5). In such 

 cases the trailing flagellum is perhaps the chief 

 organ of propulsion, acting as a pulsellum, while 

 the forwardly-directed flagellum or flagella may 

 function more as tactile organs or feelers than 

 as locomotor organs. The flagellum may also 

 serve as an organ of temporary attachment in 

 some cases, especially in parasitic flagellates ; 

 it then often exhibits at its distal extremity a 

 distinct bead-like swelling or enlargement, 

 doubtless of adhesive nature. Such terminal 

 enlargements are sometimes seen, however, in 

 free-swimming forms. 



There are many grounds for assuming the existence 

 of a gradual transition from flagella to pseudopodia, 

 and especially to the slender axopodia seen in 

 Heliozoa, etc. In organs of each kind the type of 

 structure is essentially similar, an axis of firm elastic- 

 nature, which is pushed out from the endoplasm, in 

 many cases from a basal granule of centrosomic nature 

 (p. 82). and is covered over by a sheath of contractile 

 fluid ectoplasm. The difference between them is one 

 of degree, the axopodia being relatively shorter in 

 proportion to their thickness, and consequently less 

 flexible, but the nutating and bending movements 

 seen in axopodia are essentially similar in type to 

 those manifested by flagella. The Heliozoa are con- 

 nected with the Flagellata by transitional forms which 

 indicate that their pseudopodia have arisen as 

 modifications of flagella (p. 248). Goldschmidt, who 

 discusses the whole question (41, pp. 116-122), de- 

 scribes in a Cercomonas-like flagellate the shorten- 

 ing of the flagellum, and its transformation into 

 a pseudopodium which swings to and fro. A 

 flagellum may be considered as having arisen by 

 modification and specialization of an axopodiurn, 

 and as capable in many instances of reverting to 

 that type of organ. (Compare also p. 465, infra.) 



(3) Cilia are slender, thread-like extensions of the ectoplasm which 

 differ from flagella mainly in three points : they are as a rule much 

 shorter relatively to the size of the body; they are present 

 usually in much greater numbers, and in their most primitive type 



* For a detailed description and analysis of these movements, see Delage and 

 flerouard (6), pp. 305-312. 



FIG. 25. A n isonema 

 grande, ventral view, 

 showing the " hetero- 

 mastigote " arrange- 

 ment of the flagella. 

 a.f., Anterior flagel- 

 lum ; p.f., posterior 

 trailing flagellum ; 

 8, oesophagus; c.v., 

 contractile vacuole 

 surrounded by a 

 number of feeding 

 vacuoles; N., 

 nucleus ; an., anus 

 (cytopyge). After 

 Stein. 



