394 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



(the fio-m e having been sketched from an alcohoUc specimen), but its position and the 

 marginal papillae may be seen. When the foot is contracted, as in the figure, the 

 hiteral edges of the disc are brought together ventrally, thus making a crease fi^om 

 before l)ackward tlirough the middle of the disc. The disc may be so fuUy expanded 

 that the crease sometimes disappears, but this does not often happen. 



The most usual form of foot in lamellibranchs is that typically shovvn by Venus 

 (Fig. 96, PL xciv). It is flattened fi'om side to side and extends in this case along 

 nearly the entire ventral surface of the visceral mass (/). Its anterior end is plough- 

 share shaped and is greatly protrusible. Instead of being somewhat flat on its ventral 

 surface, it is more or less sharp or heel-hke (seen in vertical section at/, Fig. 13). In 

 Mya the foot is much compressed laterally, and projecting anteriorly to the body, is 

 slightly sharper above than below (in vertical section at /, Fig. 23, PI. Lxxxii). It does 

 not extend along the ventral side of the body, or \isceral mass, as in Venm, but occu- 

 pies a position more like that of MytUus and Pecten. 



The foot of Mytilus is an entirely muscular, tongue-Mke organ, flattened dorso- 

 ventrally, concave al)ove and convex on its lower surface (Fig. 42/). In Pecteii it is 

 relatively much smaller, being a short cyhndrical projection fi^om the anterior end of 

 the visceral mass. In Ostrea and Awomia, which are fixed forms, the foot has entirely 

 disappeared. 



The foot serves a number of difi'erent piu'poses, but is generally used as a burrow- 

 ing organ. The end of the foot is protruded as a long, narrow tongiie, which digs into 

 the sand with a worm-like moA'ement, keeping the shell closed as much as possible. 

 When it has penetrated to some depth it expands at the end, the reti-actor muscles 

 come into play, and the whole animal is gradually pulled beneath the smface of the 

 sand. The forms with the ventral disc are very active. YoUlin, when bmying itself, 

 makes a sharp point of the anterior part of the folded disc of the foot and very rap- 

 idly burrows this into the nuid. The disc is now widely expanded, forming a firm 

 anchor, the foot-retractors contract and draw the body down to the end of the foot, 

 in this way quickly covering it. Solenomya has the same habit, and also often swims 

 rapidly through the water by using its powerful foot as a paddle. It is stretched out 

 anteriorly, the disc opened, and a rapid backward stroke is made. This is repeated 

 with great rapidity. One or two lamellibranchs have the habit of creeping on the 

 ventral side of the foot, as in gasteropods. but, as far as I have observed it, in the 

 adult of one very small form and in the young of Mytilus and Pecten, it is done in a 

 very imperfect way, the animal frequently being unable to maintain its erect position 

 when crawling over a smooth surface and falling over on its side. 



The foot of many lamellibranchs, as that of Venus or Anodon, is mnde up of muscle 

 fibers, which are in-eguhirly distributed vertically and horizontally, leaving every- 

 where spaces which are in connection with the vascular system. Blood being forced 

 into these spaces by the heart, causes the extension of the foot. The general direction 

 of the fibers in the foot is indicated in Fig. 13, PI. LXXXI, /. In this section a sharp 

 separation occurs between the foot and the genital gland above (g). Just below 

 this the foot shows many transverse fibers {mus). From this region, too, three prin- 

 cipal bands of muscle fibers extend down toward the ventral side of the foot. Their 

 contraction, probably, aids the foot-retractors in drawing the foot up close to the 

 visceral mass. Farther back (Fig. l-i), in the region of the posterior end of the stom- 

 ach, the sexual gland forces its way for a considerable distance down between the 



