51 



brings the epithelium of the fused interlamellar surfaces 

 of the filaments more in a plane ^\ith 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, the}'' 

 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 ffig. 

 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, 

 N. hr.). 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 ; 



