is severed and the anterior portion is left intact, the animal will 

 not close its shell. If the shell valves are pressed together the 

 muscle will not hold them, but they separate immediately when 

 they are released. The meaning of this is not clear. I feel sure 

 that it is not due to severing any nerve as the muscle has been 

 carefully scraped from the shell with the same results. That 

 there is some explanation that investigation will reveal seems 

 likely. My own work has for the most part been carried on at 

 some distance from the seashore, and the opportunity to inves- 

 tigate the action of the muscle has not yet presented itself. 



Other prominent muscles of the body beside the usual inter- 

 lacing muscles of the body wall are the radial and circular 

 muscles of the margins of the mantle (fig. i6, rpm. and cpm.) 

 the muscles of the suspensory membranes of the gills (fig. 20, 

 sm.) and the retractor muscle of the foot (fig. 10, fm.) which is 

 here confined to the left side. All of these are described in con- 

 nection with the organs with which they are associated. 



EXCRETORY ORGANS. 



These organs lie just anterior to the adductor muscle, against 

 which they are flattened, between the visceral mass and the sus- 

 pensory membranes of the gills (figs. 12 and 20, e.). Each 

 organ forms an elongated sac like body that runs from the 

 extreme latteral prolongations of the pericardium ventrally, 

 around the adductor muscle, and opens into the mantle chamber, 

 above the gills and about one-third of the diameter of the 

 adductor muscle from its ventral margin. The openings of the 

 kidneys into the mantle chamber are large, slit like, and guarded 

 by somewhat thickened lips. Not uncommonly the excretory 

 organs of lamellibranchs consist of long coiled tubes, each organ 

 being a single tube which may be nearly or quite cylindrical and 

 of nearly even diameter, or the tube may be greatly sacculated 

 or have certain enlargements. Such long coiled tubes strongly 

 suggest nephridia, and they may be looked upon as 

 modifications of this structure. Not uncommonly the organ 

 is divided into a glandular and a non-glandular portion as in the 

 fresh-water muscle, but it is usually coiled to the extent of 

 possessing at least one loop. 



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