18. 



membrane, containing a few muscle-fibers, Tflftiioh is attached to 

 the body -wall beneath the overhanging digestive gland. The palps 

 are triangular in shape and have their inner surfaces ciliated and 

 thrcwn into a series of ridges and grooves, that do not extend 

 quite to their free margins. Large blood spaces follow along 

 the bases of many of these ridges. The siipposition that the 

 large expanded palps, serve in respiration, seems probable. 



Each outer palp is supplied with a long appendage, 

 figure 3, pap, which originates from its posterior end, near its 

 dorsal margin, and can be extended far beyond the posterior mar- 

 gin of the shell. 



In young specimens, this appendage is flat and is en- 

 tirely confined to the external palp, figure 5, p'^a^p. As it 

 grows, it folds longitudinally so as to foim a groove on its in- 

 ner side, and, at the same time, twists so that it may appear like 

 a continuation of the united dorsal margins of the outer and inner 

 palps. 



Each palp appendage is supplied with longitudinal mus- 

 cles, figure 18, Im, a large nerve, p'^n, that is continued into 

 if from the dorsal margins of the palps, and a continuous blood 

 space, b s, lying beside, and morphologically ventral to it. Un- 

 der favorable conditions, blood corpuscles ma^'' be seen moving 



