24 CCELENTERATES. 



slender axis of the spool produced into a short point 

 (umbonate) at each end. The axes of these spicules are 

 directed toward the center of the gemnmle, so that, on 

 looking at the surface of the gemmule, only the radiate 

 disks on the distal end of the spicules are seen. Looking 

 at the cut edge of the outer coat of a gemmule which 

 has been divided in halves, a lateral view of some of the 

 spicules will be obtained, and their somewhat spool-shaped 

 (birotulate) outline will be recognized. 



For a better view of both skeleton and gemmule spic- 

 ules, examine some which have been isolated with nitric 

 acid and mounted separately. Draw typical spicules of 

 both kinds. 



Make an outline drawing of the whole sponge. Fill in 

 the details in a part of the drawing so as to show osteoles, 

 pores, and gemmules. 



The gemmules are reproductive bodies ; and their outer 

 coats, strengthened with beautiful spicules, however inter- 

 esting in themselves, are of value to the sponge only as 

 they form a protective covering for the protoplasmic con- 

 tents of the gemmule. 1 When, in winter, the old sponge 

 dies down, and the gemmule has been set adrift, the 

 protoplasmic cells of its interior in some favoring situa- 

 tion escape through the aperture, associate and attach 

 themselves in a relatively compact mass, and begin to 

 take food from the surrounding water, and to grow up 

 into a new sponge. 



In watching the development from gemmules, note the 

 following. 2 



1. The protoplasmic contents of the gemmule may be 

 seen to issue by amoeboid efforts, to grow and develop sar- 



1 The gemmule is formed (Marshall) by a number of free amoeboid 

 sponge cells which come together at some point in the sarcode, and secrete 

 a thin pellicle about themselves. The surrounding cells secrete the outer 

 coat of the gemmule, with its supporting spicules. 



2 For methods of preparation for this, see Appendix, p. 281. 



