46 CONCHIFERA. 



unlike those of the adult. They, therefore, pass 

 through a metamorphosis. The embryo form ap- 

 pears to differ in different genera. In some, as 

 the Unwnidce, or Freshwater Mussels, there is a 

 pair of three-sided valves, partly enveloping a 

 round mass covered with cilia, which, while in the 

 egg, continually rotated. The rotatory movement 

 soon ceases, and the embryo divides into halves, 

 each covered by a valve. Each portion of this 

 double animal has a ciliated mouth near the hinge, 

 and a proper intestine of its own. In the middle 

 of the angle formed by these halves is raised a 

 short hollow cylinder, the fo/ms-forming organ, 

 out of which projects a very long transparent 

 byssus. 



" Near the hinge a large muscle passes from 

 one valve to the other ; this, from the convulsive 

 contractions which occur from time to time, gra- 

 dually approximates the valves, which are wide 

 open when the young individual escapes from the 

 egg. These valves are trigonal and slightly con- 

 vex. One of their sides goes to form the hinge, 

 while the two remaining, which are a little arched, 

 unite at an angle opposite. With this angle is 

 articulated a prolongation curved downwards and 

 inwards, and whose convex side has several spines. 

 After their escape from the eggs, these embryos 

 are held together by their entangled tyssuses. 

 Subsequently, when the adductor-muscle has de- 

 finitely closed the valves, the embryonic halves 

 are blended together, probably by a new meta- 

 morphosis." * 



The young of Modiola and Kellia are formed on 

 an entirely different type. Their two valves, which 

 * Siebold, Anat. Invert. 200. 



