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HANDBOOK OF INVERTEBRATE ZOOLOGY. 



situated at an earlier stage. The digestive cavity now 

 becomes greatly enlarged, and cilia make their appearance 

 upon its walls, the mouth becomes connected with the 

 chamber which is thus formed, and which becomes the 

 stomach, and minute particles of food are drawn in by 

 the cilia, and can now be seen inside the stomach, where 

 the vibration of the cilia keeps them in constant motion. 

 Up to this time 'the animal has developed without growing, 

 and at the stage shown in Fig. 168 it is scarcely larger 

 than the unfertilized egg, but it now begins to increase in 

 size. The oyster reaches the stage shown in Fig. 172 in 



s St 



Fig. 171. 



Fig. 171. — A still older embryo. 

 an. Anus. a. Posterior dorsal angle. 

 b. Body cavity, st. Stomach, i. Intestine. 



Fig. 172. 



ma. Mantle, v. Velum. 



Fig. 172. — View of right side of an oyster embryo, six days old. 

 mu. Muscles. I. Liver, s. Shell. Other letters as in Fig. 171. 



from twenty-four hours to six days after the egg is ferti- 

 lized ; the rate of development being determined mainly 

 by the temperature of the water. 



Soon after the mantle has become connected with the 

 stomach, this becomes united to the body wall at another 



