The muscles of the large filaments are for the most part con- 

 tinuous with the muscular suspensory membranes. Fibers 

 extend through the inter-lamellar junctions to the free lamella 

 and serve to draw the lamellje together. Other fibers 

 extend through the inter-filamentar junctions and serve to draw 

 the filaments together and so shorten the gill. A nerve 

 (fig. 21, n.) is frequently present near the inner border (the 

 border away from the outer surface of the gill) of the filament. 

 How universally this is true has not been determined but I have 

 frequently been unable to discover such nerves. Again each of 

 a series of filaments may have its nerve. Branches from these 

 nerves have been traced into the inter-filamentar junctions and 

 presumably give out branches in turn to the filaments. They 

 probably also control the muscles of the larger filaments, inter- 

 lamellar junctions and inter-filamentar junctions. The inter- 

 filamentar muscles are especially active in gills hat have been cut 

 from the animal, and keep the gills in almost constant move- 

 ment, folding the lamellae and allowing them to straighten, as 

 they contract and relax in different portions. The large fila- 

 ments of one lamella are united to the large filaments of the 

 other lamella by rather thin membranes, the inter-lamellar junc- 

 tions (figs. 17 and 18, ilj.) that are thickened along their free 

 borders, where a large blood vessel is present. They are likewise 

 more extensive along this border so the lamellae may be sepa- 

 rated quite a distance along the upper border of the gill. The 

 shape of the membranes is such as to allow greater separation 

 than would be the case if the attachment extended straight 

 across from one lamella to the other. The bend that is made, 

 allows great freedom of movement to the free edge of the 

 lamella, which may thus be separated from the attached border 

 of the other lamella of the gill for a space of half an inch or 

 more. 



The inter-filamentar junctions (fig. 18, ifj.) are much heavier 

 along the upper margins of the lamellae than toward the free 

 edge of the gill. These junctions are very muscular, and are 

 much heavier near the large filaments than they are toward the 

 middle of the water tube (fig. 17, ifj.). They join the filaments 

 to each other and extend into the cavity of the water tubes as 

 rather prominent ridges. Each inter-filamentar junction con- 



26 



