BIOLOGY AND NATURAL HISTORY 171 



supposed to occur but rarely, both Belding and Outsell were successful in 

 securing fertilization experimentally from the sex products of one individual. 



Fertilization of the egg occurs in the water. Differential development takes 

 place during the first hours after fertilization, followed by the earliest larval 

 stage (trochophore) which resembles an annelid or worm larva. At this stage 

 the animal is capable of some locomotion with the aid of fine hairs (the cilia 

 and a flagellum). The velum, a ciliated swimming organ, soon develops, and 

 the animal is then in its veliger stage. During this stage the shell (prodis- 

 soconch) quickly develops and covers the animal, forming the typical 

 straight-hinge larva of the bivalve mollusks. Belding obtained the straight- 

 hinge stage in 17 to 40 hours. Outsell did not obtain this stage until 42 to 48 

 hours at 25° C. In four or five days some marked changes occur, such as the 

 appearance of a well developed foot, complete alimentary tract and changes 

 in the shell to resemble the adult. The characteristic ribs of the shell appear 

 when the animal has reached a size of about one millimeter (1/25 inch). 



At this stage (dissoconch) the animal begins a crawling habit and can 

 attach by means of fine threads, or byssus secreted by a gland in the foot. 

 By this means a "set" of young scallops results when attachment is made to 

 eelgrass, stones, shells, or other similar material. The young can cast off or 

 break the byssus at will to spin another. Thus, the early life consists of a 

 series of attachments and breakings free, with intervening periods of crawling 

 and swimming. The young scallop swims with its foot extended until contact 

 is made with an object to which the byssus is attached. The attachment is of 

 great significance when the young scallops are subjected to rough weather 

 conditions. If they were not attached to some object, the waters would soon 

 cast the scallops on the shores to perish. 



The ability to swim develops soon after the young reach a few millimeters 

 in size. The power of swimming makes the scallop unique among the com- 

 mercial bivalves. The whole animal is modified for this purpose, with a strong 

 light shell, powerful adductor muscle, a well-developed cartilage in the hinge, 

 and the margins of the mantle provided with infolded ridges and circular 

 muscles. Swimming is effected through coordinated efforts of the various 

 components. In swimming, the animal moves forward, hinge aft, as though 

 biting the water, rather than hinge foremost as might be expected. The mantle 

 edges close together preventing the water from escaping along the free edges 

 of the shell so that the water is forced hingeward and out of an aperture on 

 each side of the hinge when the powerful adductor muscle closes the shells. 

 Scallops are able to swim to the surface and along the surface for short 

 distances. When suddenly disturbed, the shells may snap shut and the animal 

 will dart along the bottom in the opposite direction, with the hinge foremost. 

 Other movements have been described, such as those of turning over, rotating 

 horizontally, and a sort of "sculling" movement. Although scallops are able 



