496 



bedded in connective tissue. On the side of the tentacle nearest the 

 mantle-lobe to which it is attached, inside the muscle-layer, is a large 

 nerve, a branch of the posterior pallial nerve, that can be traced to the 

 tentacle's tip. By the side of the nerve is a large blood-space, in 

 which blood corpuscles can be seen moving when the tentacle is being 

 lengthened or shortened. Extension of the tentacle seems to be ac- 

 complished by forcing blood into this space. When the muscles of 

 the tentacle contract, the blood is forced back, and the blood-space 

 may be entirely obliterated. 



In the youngest specimen collected with a dredge, the siphonal 

 tentacle appears as a little rounded knob. This knob is an out- 

 growth from the line of union of the mantle with the base of the 

 siphon, which line seems, morphologically, to be a portion of the mantle's 

 margin that was been drawn in with the developing siphon. 



Considering its origin, structure, and innervation, there can be 

 but little doubt that the siphonal tentacle is a greatly enlarged and 

 specialized marginal tentacle. Stempell (10), on page 355, objects 

 to this hypothesis, on the ground that Leda sulcata possesses a siphonal 

 tentacle and has no posterior marginal tentacles. The evidence al- 

 ready given seems to me to entirely outweigh this objection. 



The siphonal tentacle is very sensitive to mechanical stimulation 

 but its special function is not known. 



The tentacles, the siphons, the organs lying opposite the ends of 

 the stripes on the shell, and the exposed edges of the mantle are 

 all well supplied with sense papillae. 



Foot. 



The foot, Figs. 1 and 2 /", is very large and powerful in all of 

 these forms. During development the right and left sides grow out 

 into muscular flaps that lie side by side, and are capable of being 

 extended laterally so that their inner, median, sides form an almost 

 flat or arched surface. It is this part of the foot that is called the 

 sole. Although the foot is a rigid muscular organ, not possessed of 

 the usual structure of creeping organs, and known to be adapted for 

 burrowing, the supposition that it serves in creeping seems to have 

 received quite general acceptance. 



The movements of the foot of Yoldia are generally very rapid, 

 and are remarkably diversified. The movements of both species of 

 Nucula, especially N. delphinodonta, are much more sluggish. In all, 

 the movements of burrowing consist in thrusting the closed foot deep 

 into the mud, reflecting the flaps to form an anchor, and then drawing 



