OF MASSACHUSETTS. 31 



consisting of simple rotations plus revolutions. The prevailing revo- 

 lution is clockwise, but the motion is intermittent and the direction can 

 be changed at will. With the development of the flagellum, a definite 

 direction of motion arises. The animal nearly always swims with the 

 flagellum anterior, although one case has been observed where the animal 

 swam in a reverse direction for a short distance. Possibly the flagellum 

 serves to increase the speed, which becomes so rapid that it is difficult 

 to follow the animal with a microscope of 41 magnification. The motion 

 is now effected in a straight line by spiral revolutions along the longi- 

 tudinal axis of the animal. This final motion is probably the culmina- 

 tion of the previous aimless rotations. 



The Shell Gland. — The formation of the shell gland, which occurs 

 twelve to fourteen hours after fertilization, marts a decided change in 

 the character of the young scallop (Fig. 15). In the course of a few 

 hours a thin transparent shell grows slowly over the animal, until it 

 completely envelops the soft parts. At first the shell is so small that it 

 scarcely covers the whole of the animal, which can be seen swimming 

 through the water partly covered by the two thin valves. This shell is 

 formed by the secretion from the shell gland, which becomes calcified 

 at two points, forming the two valves. The hinge line at this early 

 stage is flat and straight. At the same time, with the spreading of the 

 shell, various changes of more or less importance, both in the anatomy 

 and in the habits of the young scallop, have taken place, giving rise to a 

 period in its development known as the veliger stage, perhaps the most 

 critical and important period of its existence. 



The Veliger Stage. 



"When reared in the laboratory the embryos reached the full veliger 

 (shell) stage between seventeen and forty hours after fertilization, 

 according to the temperature. Presumably the same time is true in 

 nature, although the rapidity of development varies with the external 

 conditions. The length of the veliger stage is likewise dependent on 

 temperature and environment, the usual duration being about five to six 

 days. During this period numerous changes of more or less importance 

 take place, and the late veliger is an essentially different animal from 

 the early form. It will be necessary, therefore, in describing the veliger 

 stage, with all its involved changes, to arbitrarily divide it into two 

 phases, the early (Figs. 16 and 17) and the late veliger (Fig. 18) ; and 

 in describing the anatomical changes it will be more satisfactory, after 

 a brief survey of the essential features of each phase, to trace the 

 development of the individual organs separately. 



The chief characteristics of the early veliger (Figs. 16 and 17) are: 

 (1) an equivalvular shell slightly inequilateral, without definite struc- 

 ture, with a straight hinge line, no umbones being present; (2) a velum 

 or ciliated swimming organ; (3) a primitive mouth lined with cilia. 



