30 THE SCALLOP FISHERY 



by the currents, falls to the bottom, where it remains until it acquires 

 cilia. The majority probably perish before they reach the swimming 

 stage, either through not being fertilized or because of settling on un- 

 favorable bottom. 



The swimming period is reached from nine to twelve hours after 

 fertilization, ten hours being the usual time. Little change has taken 

 place in the size of the animal, and the entire scallop is hardly larger 

 than the original egg. Development is rapid during the swimming 

 period, not so much in size as in change of form from the early swim- 

 ming gastrula to the trochosphere larva. There are three phases of 

 development in changing from the early gastrula to the advanced 

 trochosphere. (1) The animal is a mere rounded mass of cells covered 

 with cilia. (2) The body has elongated, the blastopore or primitive 

 mouth becomes more noticeable, and the cilia instead of being general 

 are confined to a special portion of the body, which later proves to be 

 anterior end (Fig. 14). In the course of two hours after phase 1, the 

 cilia on the frontal cell at the anterior end of the body elongate until 

 they attain seven-ninths the length of the body (ordinary cilia measure 

 one-fifth the length of the body), and unite to form a bundle called the 

 flagellum, which guides the swimming embryo. Ordinarily it has the 

 appearance of a single last or whip, so closely are its parts united, but 

 as many as six individual cilia have been counted in this bundle. The 

 anterior end of the animal has in the mean time become larger and 

 heavier, while the posterior half has elongated, giving the scallop a top- 

 shaped appearance. (3) The third phase is marked by another invag- 

 ination on the dorsal side of the animal, directly opposite the blastopore. 

 This is the primitive shell gland which secretes the shell. Pelseneer (9) 

 in consideiing lamellibranchiate mollusks as a class says of the shell 

 gland : " During its extension it gives rise to a saddle-shaped cuticular 

 pellicle, which becomes calcified at two symmetrical points, right and left 

 of the middle line. These two centers of calcification eventually form 

 the two valves of the shell. . ." 



The transition from the early swimming gastrula to the advanced 

 trochosphere is well illustrated by the development of the swimming 

 powers of the young scallop. As soon as the embryo has acquired cilia 

 it starts with a rolling motion, at first slowly, but later faster as it 

 increases in strength, turning over and over on the bottom of the dish. 

 This simple method of changing position is by a rotation on the longi- 

 tudinal axis which might be compared to the movement of a top before 

 it totters over. The embryo rotates in one place or hutches along in 

 random directions. The rate of this action varies greatly, anywhere 

 from five to twenty turns being counted in ten seconds. The cilia soon 

 perform the functions of swimming organs, and the little animals rise 

 through the water towards the surface, where they can get a better 

 supply of oxygen. The first swimming movement is a compound motion 



