SPONGES 115 



imbedded in the mass of gemmules? Spicules and gem- 

 mules may be better studied if a portion of the sponge con- 

 taining them be placed in weak nitric acid and allowed to 

 remain until the fleshy portion is destroyed, and afterwards 

 mounted. On the outer coat of the gemmule look for the 

 foraminal aperture through which the embryonic sponge 

 emerges. Draw gemmules and accompanying spicules. 

 Make a diagram representing a section of the entire sponge. 

 Read McMurrich's "Invertebrate Morphology" on the 

 structure and growth of sponges. 



SUMMARY. 



(i) What part of a sponge is valuable commercially? 

 Do all sponges have skeletons ? Are all skeletons of sponges 

 alike? What determines the commercial value of a sponge? 



A sponge represents a simple combination of cells which 

 differ very little from a mass of protozoans ; but while each 

 cell of the protozoan performs all the functions necessary 

 to sustain life, the cells of the sponge are slightly differentia- 

 ted so that different functions for the life of the whole may 

 be performed by cells specialized for the purpose. Although 

 the specialization is slight, and each cell, even when it is 

 enclosed in the mass, gets its own food and in a measure se- 

 cretes it own protective spicules ; yet as the sponge becomes 

 more complex, the organization increases and the cells 

 become more dependent. 



This organization of unlike cells which results from dif- 

 ferentiation of like cells gives rise to the metazoans as con- 

 trasted with protozoans. The sponges are generally con- 

 sidered the simplest form of metozoans. Read a larger 

 zoology on the life processes of sponges. 



