32 STRUCTURE 



Perhaps the most remarkable compound ciUa that have been 

 described are the comb -plates of the ctenophore Mnemiopsis 

 studied by Afzelius (1961a). Each comb -plate of Mnemiopsis 

 functions as a single unit and contains several hundred thousand 

 cilia hexagonally packed in 80 to 100 rows, each of several thousand 

 cilia. Again there is no evidence of any membrane or sheath 

 surrounding the compound structure, nor are there any projections 

 of the ciliary membrane, but the cilia show an unusual internal 

 structure which may be important in uniting cilia in rows parallel 

 to the longer sides of the comb-plates. The cilia within each row 

 are very close together (with a separation of as little as 30 A), and 

 two peripheral fibrils of each cilium (numbers 3 and 8) are 

 connected to the ciliary membrane by a longitudinal lamella about 

 80 A thick and 500 A long (PI. Vb). These compartmenting 

 lamellae, as Afzelius has called them, terminate on their respective 

 ciliary membranes exactly opposite the lamellae of adjacent cilia 

 in the row, and in some cases the narrow interciliary space between 

 the lamellar endings seems to contain a dense material that might 

 cement the ciliary shafts together along these longitudinal lines. 

 There seems to be adequate evidence that these lamellae have a 

 function in holding the cilia together, for many of the cilia at the 

 ends of the rows have lamellae on their inner side where there is a 

 neighbouring cilium, but not on the free outer side. Also, where 

 cilia of a row are pulled out of line, the lamellae may be stretched 

 and distorted so that they stiJJ end opposite each other. Normally 

 the compartmenting lamellae of all the cilia of any row are more 

 or less in one line; this line also passes through both central fibrils 

 of each cilium, and is at right angles to the plane of beat of the 

 comb -plate. Ctenophores also bear cilia on other parts of the 

 body which are not compound and do not have the lamellae. 



The cilia of ctenophore comb-pjates show two further peculiar- 

 ities. At the centre of the cih'a is a mid-filament (see p. 17 and 

 Fig. 5b), which appears in some transverse sections as an approxi- 

 mately rectangular block about 180 A by 100 A. It occupies the 

 same position in relation to the central fibrils in all cilia of a 

 comb -plate. Cilia at the edge of the two long sides of the comb- 

 plates often contain a number of intraciliary vesicles with 

 membranes about 65 A thick (the same as the ciliary membrane) 

 and sometimes with several concentric membranes. 



