GRANTIA. 13 



It will not be necessary to call attention again to the various parts 

 that will show in the sections. The student should be able to recognize 

 them from each point of view. In the tangential section the incurrent 

 canals are the smaller four-sided or irregular passages that fill hi the 

 spaces between the larger and somewhat octagonal radial canals. 



Three kinds of spicules should be distinguished the long straight 

 ones surrounding the osculum, the short straight ones, oxeote spicules, 

 covering the outer surface, and the triradiate spicules imbedded in the 

 walls. How does the peculiar form of the latter kind adapt them to 

 the purpose they serve? Test the composition of the spicules by 

 running a little dilute hydrochloric acid under the cover-glass. 



Exercise 3. Draw a complete cross-section, a small area of the tangential 

 section and one or more spicules of each kind, all properly enlarged. 



Exercise 4. // living specimens of Grantia are available for study in a 

 dish of sea water, determine the direction of water currents by intro- 

 ducing a little ink. 



