THE ORGANIZATION OF THE PROTOZOA 57 



ever, serve for other functions than that of locomotion, in flagel- 

 lates as well as in ciliates. In large, stout forms of trypanosomes, 

 for example, the animal may remain perfectly still while its mem- 

 brane is rippling actively, and in that case the function of the mem- 

 brane is probably to cause currents in the fluid surrounding the 

 body, and to change and renew the liquid bathing the body-surface. 

 In such a case it has been noted that the undulating membrane 

 may from time to time reverse the direction of its movements, the 

 waves running for a time from the hinder end forwards, and then 

 for a time in the opposite direction (Minchin and Woodcock, 42, 

 p. 150). It is probable that the undulating membranes which pass 

 down the vestibule of Vorticellids can reverse their movements in a 

 similar manner, since this passage serves both for passage of food- 

 particles to the mouth and for the ejection of excreta from the anal 

 pore and the contractile vacuoles. 



The only structures found in free-living Flagellata which can be 

 compared at all with undulating membranes are the peculiar 

 " collars " found in the Choanoflagellata (Fig. 110), and also in tne 

 collar-cells of sponges. Each collar is an extension of the ecto- 

 plasm which grows up from the edge of a circular area round the 

 insertion of the flagellum, forming a membrane like a cuff or sleeve 

 surrounding the basal portion of the flagellum, but quite distinct 

 from the flagellum itself, and not formed in actual connection with 

 it like the undulating membrane of a trypanosome. The collar 

 differs further from a true undulating membrane in not being 

 energetically motile, but only slowly protrusible and retractile. It 

 has been stated, both for Choanoflagellates and for the collar-cells of 

 sponges, that the collar is formed by a spirally-folded membrane. 

 Their function appears to be that of assisting in food-capture by 

 a sessile, flagellated organism. 



(4) Contractile mechanisms in Protozoa, when they are visible, 

 take the form of so-called myonemes, minute contractile fibrils run- 

 ning in various directions in the ectoplasm, like an excessively 

 minute system of muscle-fibres. Such elements are not found in 

 Sarcodina or in the non-corticate forms of the other classes ; in 

 naked forms with amoeboid movement the ectoplasm, as has been 

 pointed out above, is only a temporary differentiation of the proto- 

 plasmic body, which can arise by conversion of the endoplasm, and 

 which can be changed back again into endoplasm. Myonemes occur 

 commonly, however, in those . Flagellata, Spurozoa, or Infusoria, 

 which owe a definite body-form to the presence of a firm cuticle or 

 cortex, representing a stable ectoplasm. The myonemes are often, 

 however, extremely fine, and sometimes escape detection in cases 

 in which we can infer their presence with certainty from the move- 

 ments or contractions of the organism or of its ectoplasm. As a 



