SEA-FISHERIES LABORATORY. 373 
deep that 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, al.p.). Owing to the presence of two forms of 
filaments the gills are known as J//eterorhabdic gills. 
Principal filaments are only developed in plicate gills, 
and always in connection with interlamellar septa, which 
Fic. 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.l.). 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 todo. There are no organic interfilamentar 
