SEA-FISHERIES LABORATORY. 388 
brings the epithelium of the fused interlamellar surfaces 
of the filaments more in a plane with the ctenidial axis. 
This means that the outer surface of the lamella tends 
to become a continuous plane surface, to which are 
attached the branchial expansions. The cavities of the 
ordinary filaments remain in their normal positions, and 
are far removed from the epithelium which once bounded 
their inner faces. Furthermore the nerves which ran 
down the inner edges of the ordinary filaments have 
become displaced with the development of connective 
tissue and are near the outer edges of the lamellae, they 
are much larger, and almost touch each other. 
The outer margin, as soon as it becomes a plane 
surface, is continuous with the ctenidial axis. The 
expansions formerly on the principal filaments are 
continued over the outer surface of the axis for a little 
way, gradually diminishing in size until the level of the 
afferent branchial vessel is reached and the vessel of the 
expansion communicates with it. 
The nerves from the various filaments, forming 
almost a lamella on each side, pass between the band of 
muscles which runs along each side of the gill axis (fig. 
45, Br.m.) and the surface, and become connected with 
small ganglia or groups of ganglion cells, from which 
nerves pass up at the sides of the afferent vessel (fig. 45, 
Br. aff.) and connect on to the ctenidial nerve (fig. 45, 
NV. br.). The presence of connective tissue and longi- 
tudinal muscles cuts off the cavity of the expansion on 
the principal filaments from the cavities of the principal 
filaments themselves, and further from all communication 
with the other filaments. 
On the inner side of the plicate gill lamellae the 
epithelial layer tends to lose its plications and become a 
plane surface by the development of connective tissue ; 
