560 BJORN A. AFZELIUS 



cross-cut. In these flagella the peripheral filaments can be seen to contain 

 subunits of different electron density. The subunit provided with "arms" 

 and "spokes" appears dark, the other one appears light. Similar findings 

 have been described from observations on some mammalian (Faw^cett, 

 personal communication) and an avian [14] spermatozoon. 



The next type of flagellum to be described is that of the sponge (M/cro- 

 cioua sp.) collar cell (choanocyte). According to Kilian [12] the choanocyte 



Fig. 2. Cross-section through four squid sperm tails (/). The left tail is sec- 

 tioned close to the centriole and is partly surrounded by the nucleus («). In this 

 flagellum and in the one to the right a proper orientation of the section has allowed 

 a detailed study of the flagellar filaments. The nine outer filaments are connected 

 to nine electron-dense accessory filaments that form a circle outside the proper 

 flagellar filaments. The nine flagellar filaments are themselves double, in that 

 they can be said to consist of two subunits, one having a light and the other a dark 

 appearance. Magnification 81 250 x . 



flagellum works with a regular flagellar beat when the collar is expanded 

 but with a beat similar to that of cilia when the collar is retracted. P'igure 3 

 shows at low magnification a section through a chamber lined with collar 

 cells. The marked area in this picture is further enlarged and is shown in 

 Fig. 4. Two notable features characterize this flagellum with regard to its 

 fine structure: (i) There is a marked difference in size between the two 

 subunits of each of the nine peripheral filaments. The larger subunit is 

 that which is provided with arms ; a reverse proportion has been found in 

 some multiflagellated protozoa [7]. (2) There are thin indistinct hairs 

 lining two sides of the flagellum (marked h). These hairs are roughly in a 

 line parallel to the two central filaments. The appearance is similar to that 



