106 



F. M. BALFOUR. 



After invagination the cilia of the entoderm cells can no longer 

 be seen, and are probably absorbed, and their disapjjearaiice is 

 nearly coincident with the complete obliteration of the blasto- 

 pore, 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 apppear- 

 ance 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 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 sur- 

 face is flattened, and though surrounded by a ring of four-rayed 

 spicules is itself free from them. At this extremity a small per- 

 foration is formed leading into the gastric cavity which rapidly 

 increases in size and forms an exhalent osculura (os). A series of 

 iuhalent apertures are also formed at the sides of the cylinder. 



Fig. 3. 



Tlie young of Sycandra raphanus shortly after tlie development of tlie 



Spicula (copied from Sciiulze). 

 View from the side ; b, view from the free extremity ; os. oscuhim ; 



ec. ectoderm ; en. entoderm composed of collared ciliated cells. 



The terminal osculum and lateral pores are represented as oval white 



spaces. 



