THE MASTIGOPHORA 261 



or Craspedomonads (Fig. 110) a peculiar structure occurs, known 

 as the " collar," a delicate protoplasmic tube or funnel which 

 arises along a circular base-line of which the insertion of the flagellum 

 is the centre, and so forms a cup, sleeve, or collar-like structure 

 surrounding the flagellum for about a third or a half of its length. 

 It is stated, both for Choano flagellates and for the very similar 

 collar-cells of sponges, that the collar is a membrane folded in a 

 spiral manner, its insertion running along the body and round the 

 base of the flagellum ; but the spiral structure is not easy to make 

 out. The Choano flagellates are sedentary forms which, if set free 

 temporarily from their attachment, swim with the flagellum 

 directed backwards, doubtless the mechanical result of the presence 

 of the collar. The function of the collar is probably connected 

 with the capture and absorption of food-particles wafted towards 

 the body by the flagellum. The collar is retractile, but is not capable 

 of active movements such as are seen in an undulating membrane. 



The organs of nutrition must be considered in connection with 

 the four modes of life already mentioned. 



(a) In holozoic forms the organism captures and ingests other 

 organisms of various kinds. In some forms the ingestion of food- 

 particles may take place at any point on the body surface ; examples 

 of this are the amoeboid forms, such as Mastigamosba, which capture 

 their food by means of their pseudopodia, like an amoeba ; the holo- 

 mastigote genus Multicilia (Fig. 113) ; the parasitic Lophomonas 

 (Fig. 45), and possibly others. But in most cases food-particles 

 are ingested at the base of the flagellum, the spot towards which 

 they are propelled by the activity of the flagellum itself. There 

 may, however, be no special aperture for food-ingestion, particles 

 which impinge upon the soft protoplasmic body being simply 

 absorbed directly with formation of a food-vacuole. With a more 

 advanced type of organization, a special aperture or cytostome for 

 the ingestion of food-particles is found at the base of the flagellum. 

 The cytostome may be a simple aperture leading through the cuticle 

 directly, or by means of a funnel-shaped depression, into the proto- 

 plasmic body, or it may, in more highly organized forms, lead into 

 a special tube, termed an " oesophagus " or " cytopharynx," which 

 receives the evacuations of the contractile vacuoles, and serves for 

 excretion as well as ingestion (Fig. 84). In any case the oesophagus 

 ends blindly in the fluid endoplasm. There is no special anal aper- 

 ture for expulsion of faecal material, which is expelled at any point 

 of the body-surface in primitive forms, or through the oesophagus 

 and cytostome in those more highly organized. 



(6) In holophytic forms the organs of nutrition are those of the 

 plant-cell (p. 188) namely, chromatophores, or corpuscles contain- 

 ing chlorophyll or allied pigments ; pyrenoids, small glistening bodies 



