34 THE SCALLOP FISHERY 



to swim rapidly in a straight course. Only when the velum can be 

 completely extended does the larva attain full swimming powers. 



When the velum is fully developed the animals become rapid swim- 

 mers, and can be found in great numbers through the water, more 

 especially near the surface, where they can be taken in a net of silk 

 bolting cloth. When placed in a glass aquarium, if left undisturbed, 

 they can be seen by the naked eye as white specks as they swim through 

 the water. If the dish is subject to any sudden jar, such as a sharp 

 tap with a pencil, the young scallop quickly pulls in the velum and 

 settles to the bottom with closed shell. After a brief interval the ani- 

 mal extends the velum with a hesitating jerky movement, until it is 

 fully expanded, and then resumes swimming. The usual direction is 

 with the velum ahead, the cilia on the edges lashing with a rowing 

 motion which propels the animal in the same manner as a boat is 

 propelled by a man seated in the bow. There is also a turning motion, 

 which whirls the larva anterio-posteriorly in either a clockwise or anti- 

 clockwise direction. 



The Foot. As the animal passes into the late veliger stage the 

 swimming powers of the velum degenerate, while the foot with its 

 ciliated tip becomes the only organ of locomotion. The footed larvaa 

 swim by a kicking movement of the foot. It is natural to suppose that 

 there is a transitory stage where both the velum and the foot are used. 

 The foot, the most useful organ of the young scallop, makes its appear- 

 ance in the prodissoconch stage, and for a long time serves as means 

 of locomotion for the animal. It is a long, flexible organ, made up of 

 both longitudinal and circular muscles, and entirely covered with fine 

 cilia. On its tip or distal end are long cilia, comparable to the little 

 tuft or cluster posterior to the mouth in the early veliger. The long 

 cilia are at first useful in swimming, but as the animal becomes larger 

 they become relatively less important. The tip of the foot is slightly 

 cleft, as is shown for an older scallop (Fig. 27). On both sides of the 

 foot in a median position are two vestibules, with several small granules 

 rotating inside. These are the otocysts or organs of equilibrium. On 

 the dorsal side of the foot, one-third the distance from the proximal 

 end, is a prominence with a cleft opening, the byssal gland, the function 

 of which has not culminated at this stage. The foot is capable of great 

 extension by the contraction of the circular muscles, and is drawn in 

 by the contraction of the longitudinal to lie in its normal curved posi- 

 tion within the shell (Fig. 18). 



The Adductor Muscles. The primitive veliger has but a single ad- 

 ductor muscle, the anterior. In the dissoconch stage, the posterior 

 adductor is the only one present, the anterior having disappeared. As 

 is stated by both Jackson (4) and Drew (1), there must be an inter- 

 mediate stage where both are present. I have obtained no actual proof 

 of this, but in all probability a dimyarian stage, i.e., having two mus- 

 cles, must have been reached in the course of development. 



