122 



INVERTEBRATA 



CHAP. 



Running round the edge of the larva under the prototroch a nerve- 

 ring has been detected. 



The development of the Pilidium is now complete, and it swims 

 about at the surface of the sea feeding on microscopic organisms 

 which are whisked into its mouth by the action of its cilia. After 



,pr 



cam. 



FIG. 97. A Pilidium larva shortly before its metamorphosis. (After Metschmkoff. ) 



Letters as before. In addition, a.im, anterior imaginal disc ; e.s, rudiment of cephalic slit; 

 pr, rudiment of proboscis ; p.im, posterior imaginal disc ; oes, oesophagus. 



about two weeks it begins its metamorphosis. This has been described 

 by Metschnikoff (1869) and Salensky (1886). 



On the flattened under side four ciliated invaginations of the 

 ectoderm are formed. These are termed the amniotic invaginations, 

 and their deeper portions are the imaginal discs. Two of these 

 invaginations are situated opposite one another on the right and left 

 sides of the animal respectively, in front of the mouth, and two 

 others are similarly situated behind the mouth. Each of them grows 

 and deepens, extending upwards over the surface of the globular 



