G. A. DREW ON YOLDIA LIMATULA, SAY. 9 



function, for which its structure so admirably fits it, is to burrow in 

 soft mud.* 



BYSSAL GLAND. 



FlGUEES 4 AND 5, bg. 



In none of the specimens that came under my observation, did this 

 organ seem to be functional. The youngest specimen collected with a 

 dredge shows a small pouch, lined with epithelium, which opens into the 

 posterior end of the groove that separates the muscular flaps of the foot. 

 The gland receives a rather large nerve from each pedal ganglion, and 

 it often contains a few fine threads. These threads have never been seen 

 protruding from the duct of the gland. 



ALIMENTARY CANAL. 



FIGURES 4 AND 5. 



The rather broad and long oesophagus opens into the dorsal end of 

 a capacious stomach, Fig. 4, sto., which extends from near the dorsal 

 margin of the animal, well into the foot. The stomach is divided 

 transversely by a well-marked constriction, and internally by a 

 prominent ridge, formed by elongated epithelial cells. Above this ridge 

 the epithelial cells are rather slender, and stain deeply. Below the 

 ridge the epithelial cells are broader and stain less deeply. 



From the ventral end of this portion of the stomach, the intestine 

 bends back along its posterior side, ascends to a point just beneath the 

 pericardium, makes a forward turn, comes very near the surface on the 

 right side, follows the edge of the genital mass and digestive gland 



* Specimens of Yoldia limatula, Say, Nucula proxima, Say, and Nucula delphino- 

 donta, Mighels, have been observed for hours together, in smooth and rough-bottomed 

 dishes, in dishes containing sand, and in dishes containing the soft mud in which the 

 animals normally live. They all burrow with rapidity and in the same manner, but 

 in no case was a specimen observed to creep, even for the shortest distance. The 

 expanded foot of N. delphiuodonta is very large in proportion to the size of the 

 animal ; the movements are comparatively sluggish, and the almost spherical shell is 

 raised or turned over with but little difficulty, but the movements of the foot are 

 always such as are used in burrowing. Furthermore, in none of these forms does the 

 foot correspond in structure with that of known creeping organs. 

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