the branchial and the cloacal chambers as the pressure of the 

 water in the cloacal chamber caused by the rapid closing of the 

 shell would be certain to injure the gills. At such times the gills 

 are kept from injury by the contraction of the muscles of the 

 interlamellar junctions, so the lamellse of each gill are drawn 

 together and by the contraction of the suspensory membranes 

 of the gills which draw them away from the margins of the shell 

 and keep them from being crushed. No doubt the arrangement 

 of the gills in this form is to be explained by its exceptional 

 habits. 



As before stated the two lamellae of a gill are attached to each 

 other at intervals corresponding to the striations that run the 

 width of the gill. These lines of attachment (fig. 17, ilj.), the 

 interlamellar junctions, form complete partitions so the space 

 between the lamellse is divided into a series of tubes, the water 

 tubes, that are closed, except for minute openings in the sides, 

 (io.) the inhalent ostia, and where they open into the cloacal 

 chamber. Each tube extends from the free border of the gill 

 (figs. 18 and 20). where it is closed by the joined lamellae, to 

 its opening in the cloacal chamber, and is bounded by the 

 lamellae and by the interlamellar junctions. Of these water 

 tubes there are several hundred in the length of each gill. 



Each lamella is composed of a series of delicate filaments, (fig. 

 17, gf.)the gill filaments, that run the width of the gill parallel 

 to the more prominent striations. These filaments are of two 

 kinds, large ones concerned in the formation of the inter-lamellar 

 junctions, and small ones. They are all connected at intervals 

 by cross bars, (ifj.) the inter-filamentar junctions, that run at 

 right angles to them. 



The crossing bars (the filaments and the inter-filamentar 

 junctions), leave spaces, the inhalent ostia (fig. 17, io.) between 

 them, that are the openings that have been referred to as 

 leading into the water tubes. The inhalent ostia are much larger 

 and more regular in the scallop than in most other lamelli- 

 branchs, as the lines of fusion that form the inter-filamentar 

 junctions are not nearly as extensive as in most other forms. 



Usually the filaments of one lamella are continuous with those 

 of the other at the free margin of the gill, so it is quite possible 

 to trace a filament from the suspensory membrane down one 



22, 



