878 



THE AMERICAN NATURALIST [Vol. XL- 



They consist of three main filaments arising from the ventral rec- 

 tal wall, each about 400 /x long and 120 wide at the base, tapering- 

 to 7')// near the middle and to 60// at the distal end, and each fur- 

 nished with many finger-like branches arising from their lateral 

 and ventral walls (see Fig. 1, B.). 



As might be expected from their morphological relationship,. 



the gills arc composed of the same three layers as is the rectum;, 

 the intima on tlie surface, the epithelium next, and the basement 

 membrane adhering closely to the epithelium and lining the gill 

 cavity. The intima is exceedingly thin, perfectly transparent and 



