48^ 



Tills condition of the enbr^ro is reached in from 90 to 

 120 hotirs. It now stops swinmlng, settles to tlie bottom, the 

 apical cilia shrivel, and the test cells break apart and freqiient- 

 ly go to pieces at once. The process of casting the test gener- 

 ally begins with the posterior row of cells and woiks forward. 

 It generally takes but a very few minutes. Casting includes, 

 beside the test, the stalk which extends from it to the cerebral 

 ganglia, the apical plate and its connection, and the stomodae^im 

 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 \ip, raaj not sever tlieir 

 connection for a half hour or more, figure 34, but they finally 

 break off Just belovr the cerebral ganglia, and casting is com- 

 plete. 



TJntil about the tine that the test is cast, the shell 

 valves remain gaping ventral ly and the tissues of the body ex- 

 tend completely across' from valve to valve. ItVlien the sliel 1. is 

 finally closed by the contraction of the adductor muscles, there 

 is no longer sufficient space for t2"ie organs in tlieir original 

 position. The principal change^ of importance due to this crowd- 

 ing, takes place in the pouches of the digestive glands, which 

 are forced into the space dorsal to the alimentary canal, and, in 



