42 INVEKTEBKATA CHAP. 



cesses and adhere to the substratum ; the process of attachment of 

 the larva to the substratum is known as fixation. 



The larva is thus converted into a closed cylinder, the wall of 



FIG. 20. A late stage in the metamorphosis of the Ascou stage of Sycandra raphanus 



into the adult condition. (After Maas. ) 



Letters as in Fig. 19. 



which consists of an outer layer of flattened cells, and an inner layer 

 composed of ciliated cells, on each of which the collar soon makes its 

 appearance. This collar is characteristic of the cells lining the flagel- 

 lated chambers in all Porifera. Between the two layers a layer of 



