No. 505] 



THE CANADIAN OYSTER 



37 



importance to the animal is the foot (Figs. 7, 8, 9), a 

 structure which I claim the privilege of having first rec- 

 ognized. The adult oyster is normally a quiescent, ses- 

 sile animal, having its left valve solidly cemented to a 

 rock or another shell. Under these conditions a creeping 

 foot, such as is possessed by a clam, a mussel, or a quohog, 

 would be of no service to the oyster, which in fact lias 

 none. Influenced, no doubt, by this difference in the 

 adults, zoologists have been accustomed to think of the 

 oyster larva as being vastly different from other bivalve 

 larva?, and repeatedly state that it has no foot, a miscon- 

 ception which justifies the view with which I started out, 

 viz., that plankton stages of oyster larvae have been 

 neglected, embryologists jumping from early veliger or 

 phylembryo to late prodissoconch or early nepionic 

 periods. The foot, at the period we are studying, is well 

 developed, and is a most capable organ, by means of 

 which the animal can creep rapidly about and forcibly 

 flop its heavy shell from one side to the other. When 

 extended it is a long, slim, ciliated, muscular outgrowth 

 from the middle of the ventral surface of the body of the 

 larva, behind the velum. Its lower or posterior surface 

 sometimes appears flattened or even grooved lengthwise 

 (Figs. 18, 19, 21), and at a short distance from the base 

 of attachment there is a heel-like projection (Figs. 8, 9, 

 13, 14, 21) which doubtless contains the opening of the 

 byssus gland. When quiescent the foot is shortened, re- 

 tracted and closely tucked away behind the velum and 

 between the gills, but it can stretch so far as to perforin 

 feeling movements over all parts of the body within the 

 shell and even bend up along the outside of the shell. 

 I have no doubt that at the end of the free-swimming 

 period, when the velum fails as an organ of locomotion 

 and the larva has to remain at the bottom, the foot then 

 proves to be of greatest service in freeing the little animal 

 from overwhelming sediment, creeping on to a solid sub- 

 stratum, clearing a suitable place for fixation, and per- 

 haps furnishing a transitory byssus. 



