664 



MORPHOLOGY AND SYSTEMATIC 



After invagination the cilia of the entoderm cells can no 

 longer be seen, and are probably absorbed, and their disap- 

 pearance is nearly coincident with the complete obliteration of 

 the blastopore, an event which takes place shortly after the 

 attachment of the larva. After the formation of the structureless 

 layer between the ectoderm and entoderm, calcareous spicules 

 make their appearance in it as delicate unbranched rods pointed 

 at both extremities. The larva when once fixed rapidly grows 

 in length and assumes a cylindrical form (fig. 3, A). The sides 



FIG. 3. 



The young of Sycandra raphanus shortly after the development of the spicula 



(copied from Schulze). 



A. View from the side; B, view from the free extremity; os. osculum; <r. ectoderm; 

 en. entoderm composed of collared ciliated cells. The terminal osculum and 

 lateral pores are represented as oval white spaces. 



of the cylinder are beset with calcareous spicules which project 

 beyond the surface, and in addition to the unbranched forms, 

 spicules are developed with three and four rays as well as 

 some with a blunt extremity and serrated edge. The extremity 

 of the cylinder opposite the attached surface is flattened, and 



