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 the 

 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 flagellurn, 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 eiidoplasm, and 

 which can be changed back again into endoplasm. Myonemes occur 

 commonly, however, in those Flagellata, Sporozoa, 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 



