41 



deep tliat their surfaces are situated at right angles to the 

 plane of the axis. The plication is least near the free 

 ventral edge of the lamellae. Further, the filaments are 

 not uniform in size or structure, those occupying the 

 bottom of the furrows between two successive plicae 

 being larger and known as Principal filaments (figs. 21, 

 22, Fil. p.). Owing to the presence of two forms of 

 filaments the gills are known as Heterorhabdic (/ilh. 

 Principal filaments are only developed in i)lifate gills, 

 and alwaj^s in connection with interlamellar septa, which 



Fig. 2. Section through base of gill filaments, x 50. 



are organic sheets of membrane connecting the ascending 

 and descending portions of these filaments (fig. 22, 

 Br. g. I.). In very young specimens the principal 

 filaments and interlamellar septa are not developed, both 

 being secondary differentiations. 



The adjacent ordinary and principal filaments are 

 connected in P. maximus and P. opercularis by stiff cilia 

 only, arranged in patches known as ciliated discs. The 

 cilia of two opposite discs interlock just as two brushes 

 can be made to do. There are no organic interfilamentar 



