SOME PROBLEMS OF CILIARY STRUCTURE AND CILIARY FUNCTION 563 



of so-called " flimmcrfiayella " which ha\e been described in plants by 

 Manton [13]. 



We will now turn to cilia. One animal seems to ha\e been used more 

 often than any other in studies of ciliary movement, namely the mussel, 

 Mytilus edulis. The gills from this animal seemed suitable for examination 

 in the electron microscope although there are several different types of 

 cilia in a mussel gill. Figures 6, 7 and 8 represent some of the types 

 present. In Y\g. 8 the section passes near the tips of the gill cilia, and it 

 can be seen that the peripheral filaments are single. As in sea urchin sperm 

 tails there is no evidence here that the individual filaments join close to the 

 tip. In Fig. 7 which represents another type of gill cilia there is on the 

 other hand a top plate in which the eleven filaments fuse. A similar distal 

 fusion of the peripheral filaments have been described by Rhodin and 

 Dalhamn [i^] in cilia from rat trachea. Figure 6 represents a longitudinal 

 section through the basal parts of two " laterofrontal cilia". The peripheral 

 fibres can be followed from their more distal parts down through the 

 "basal plate" (arrow). Thev terminate at some distance below this plate 

 (cf. ref. [6]). 



The last two figures (Figs. 9 and 10) are cross-sections of a unique type 

 of cilia which constitute the ctenophore swimming-plate [Miieiiiiops/s 

 h'idyi). The cilia are very long and a great number of them are fused 

 together. Their fine morphology is equally unique, and the filament 

 pattern can be described as 9 -I- 3 in contrast to the usual 9 + 2 (Fig. 10) [2]. 

 There are ridges in the cilia which join two of the peripheral filaments to 

 the cell membrane — and in many instances seem to connect filaments in 

 neighbouring cilia through a similar substance between the ciliarv mem- 

 branes (arrows in Fig. 9). These ridges presumably represent the morpho- 

 logical equivalent of the phenomenon of ciliary fusion. 



We have now some information on the structure of cilia and fiagella, 

 and we have some information on their function. We have two types of 

 information, but these two types do not seem to fit together well. There is 

 no simple answer to the question of ciliary movement. At the present time 

 one is tempted to propose temporary working hvpothesis by finding 

 analogies in other systems that are better understood. The most obvious 

 analogy is the contraction of a muscle. Perhaps the filament-sliding 

 hypothesis of Huxley and Hanson [11] may ser\"e as a model. Bio- 

 chemically the work of muscles and the work of cilia and fiagella appear 

 similar although not identical [8, 10]. 



The bending of a cilium or a flagellum must consist of a contractile 

 element as well as of an element capable of resisting compression, an 

 elastic backbone [9]. It seems likely that the nine peripheral filaments are 

 contractile units. The "arms" have a certain resemblance to projections 

 on the myofilaments [i]. As the nine filaments are continuous throughout 



