^42 Comparative Animal Physiology 



Cilia are essential to the normal circulation of the body fluids by some 

 annelids such as the pelagic polychaete, Tomopteris, and the sipunculoids 

 and echiuroids,^^' "'^ where a true circulatory system is vestigial or absent. 

 The coelomic fluids are caused to circulate in a very effective manner by the 

 action of specifically organized and directed ciliary tracts. The coelomic fluids 

 of starfish are circulated in a similar fashion, the action of cilia even effec- 

 tively providing for circulation within the dermal papillae. The cerebrospinal 

 fluid of the \'ertebrate has been reported to be circulated by action of cilia of 

 walls of the ventricles." 



In numerous instances among animals, particularly in the tubules of 

 nephridia and kidneys and in ducts of the genital system, cilia are respon- 

 sible for facilitating the passage of the materials normally conducted. 



The foregoing brief summary of the distribution and general roles of cilia 

 indicates the abundant occurrence and varied functional significances of 

 these organelles. Although they are by no means able to produce movements 

 of the power and conspicuousness of rhose produced by muscle contraction, 

 there are abundant instances where cilia normally are highly essential func- 

 tional constituents of the organism. And in numerous of these instances they 

 perform functions in a far more efficient and effective fashion than could 

 probablv be done b\ any conceivable typical muscular mechanism. 



The Structure of Cilia. Cilia in Hving cells show a great uniformity of 

 structure and organization. The cilium appears, optically, quite homoge- 

 neous. In appropriately treated ciliated cells the cilium can be seen to com- 

 prise a sheath containing an axial filament. The sheath may be circular or 

 ovoid in cross section. The contained axial filament may pursue a straight 

 course through the sheath or may follow a spiral one. The axial filament 

 not uncommonlv extends distallv beyond the limits of the sheath. 



The flagellum of some flagellates, e.g., Eiiglena, has attached to the 

 sheath a series of diagonally oriented rodlets or mastigonemes which give the 

 flagellum a feathery appearance.-^' ^■•- "^- '^^ Flagella of this structure are va- 

 riously called '"feather-type," "ciliary," or "stichonematic," in contrast with 

 the simpler whiplike type. The proximal end of the axial filament is invari- 

 ably associated with a basal granule, which is believed to be derived from the 

 cell centrosome. Many cytologists have described systems of fibrils proceeding 

 from the basal granules to the \'icinity of the cell nucleus. 



The flagellum of Euglena has been shown, after treatment with osmic 

 acid, to be resolvable into four longitudinal unbranched fibrils spirally 

 twisted over one another.^" A similar ultrastructure has been disclosed with 

 the aid of electron micrographs (Fig. 242) for the cilia of Paramecium. 

 Frontunia, CoJpidiinn,''''--^^-^^ and the ciHa of the clam gill. In the tail of 

 the squid sperm the number of these longitudinal fibrils is 9 or 10, and this 

 number shows verv little variation among numerous sperm cells from this 

 animal. 



The foregoing observation, together with the fact that cilia show positive 

 form and intrinsic birefringence,*'"*' *'•' all indicate that longitudinally ori- 

 ented submicroscopic micelles compose the cilium. In this characteristic, 

 therefore, cilia have much in common with contractile fibrils of muscle tissue. 

 Cilia often appear as compound organelles composed of numerous simpler 

 units. The cirri of Euplotes or the large cilia of Nephthys gills may be re- 



