26 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



the ventral margins of the foot, but its posterior end, Fig. 61, std, 

 lies free between the lobes of the mantle, sometimes almost, if not 

 quite, in contact with the test, sometimes raised well above it. The 

 anterior end of the mid-gut is enlarged to form the stomach, which 

 communicates with the pouches of the digestive glands. Most of the 

 tissue dorsal to the intestine has been used in forming the mantle, 

 which is now separated from it by a large space. This space extends 

 to and around the anterior adductor muscle anteriorly, and nearly to 

 the margin of the shell posteriorly. The cerebral, pedal, and visceral 

 ganglia, with their commissures, have been formed. The otocysts, 

 Fig. 61, ot, have apparently been entirely closed off, and each contains 

 an otolith, which stains deeply with hjematoxylin, and, a little later, 

 plainly shows concentric structure. As the otocysts have never been 

 open to the exterior, development having taken place inside a closed 

 test, the otoliths cannot be foreign particles. 



The only places where the test is attached to the body of the 

 embryo, lying inside it, are around the blastopore, along the sides of 

 the apical plate, and around the opening of the unpaired pouch, Fig. 

 33, r. The embryo still swims rapidly, but has periods of rest. 



This condition of the embryo is reached in from 90 to 120 hours. 

 It now stops swimming, settles to the bottom, the apical cilia shrivel, 

 and the test-cells break apart and frequently go to pieces at once. 

 The process of casting the test generally begins with the posterior 

 row of cells and works forward. It generally takes but. a very few 

 minutes. Casting includes, besides the test, the stalk which extends 

 from it to the cerebral ganglia, the apical plate and its connection, 

 and the stomodaeum from the blastopore to the position of the definitive 

 mouth. These parts, together with a few adhering test-cells that are 

 no longer stretched flat, but have rounded up, may not sever their 

 connection for a half hour or more, Fig. 34, but they finally break off 

 just below the cerebral ganglia, and casting is complete. 



Until about the time that the test is cast, the valves of the shell 

 gape ventrally, and the tissues of the body extend completely across 

 from valve to valve. When the shell is finally closed, by the contraction 

 of the adductor muscles, there is no longer sufficient space for the 

 organs in their original position. The principal change of importance 

 due to this crowding takes place in the pouches of the digestive 

 glands, which are forced into the space dorsal to the alimentary 

 canal, and, in all of the specimens examined, seem to give rise to a 



