ZOOFLAGELLATES 135 



flagellates are of particular interest to zoologists because of their 

 considerable structural similarity to the choanocyte cells of 

 sponges, and are thus considered as possible representatives of a 

 stem line leading to at least this phylum of animals. 



The first electron-microscope study of a choanoflagellate was 

 reported by Petersen and Hansen (1954) and dealt with whole 

 mounts of Codonosiga botrytis. This species has a hyaline collar that 

 is contractile and may be entirely withdrawn. In some electron 

 micrographs a definite fibrous zone bordering the flagellum 

 suggests the presence of poorly preserved mastigonemes of the 

 ochromonad type; if correct this would constitute evidence of 

 chrysomonad affinities. The collar is made up of fine individual 

 thread-like processes. 



Thin sections of the same species, studied by Fjerdingstad 

 (1961b), show that the threads resemble orderly microvilli. They 

 are 120 to 210 m^ in diameter, regularly spaced in a single ring; 

 their cytoplasm appears undifferentiated and is continuous with 

 that of the cell body. An amorphous material irregularly covering 

 the limiting membrane of collar microvilli and cell body is 

 interpreted as mucus. The author considers that the collar com- 

 ponents are pseudopodial and retractable. In sections it is 

 apparent that the cell is surrounded to a distance of some microns 

 up the collar by a cup-shaped envelope composed of rather dense 

 material with a suggestion of fibrillar constitution. Outside the 

 collar, well below the rim of the envelope, is a deep invagination 

 of the cell surface that Fjerdingstad considers to be the site of 

 formation of food vacuoles. According to his interpretation, 

 flagellar movements create a water current passing inward through 

 the collar, which acts as a sieve to trap food particles that then are 

 passed down its outer surface to the cell proper. The single 

 flagellum and kinetosome of Codonosiga are of conventional 

 structure. Fjerdingstad describes but does not illustrate striated 

 fibrils at the bases of the collar and flagellum. Details of cyto- 

 plasmic structure are not discussed and it is not possible to 

 determine from the published pictures whether the mitochondria, 

 for example, are microtubular or crista!. 



That the collar of the sponge choanocyte is essentially identical 

 to the choanoflagellate collar is demonstrated in electron- 

 microscope studies by Kilian (1954, whole mounts only), Rasmont 



