THE FRESH-WATER SPONGE. 28 



almost the whole surface, and through which the water 

 enters the sponge. 



The sponge flesh is called sarcode. Press upon it, and 

 note its consistency. 



Examining it again with a lens, note that it is bristling 

 with the points of very minute spicules, which are arranged 

 somewhat in lines, so as to form a sort of skeleton, running 

 through and supporting the sarcode. These skeleton spic- 

 ules are about ^- of an inch long. 



The sarcode is permeated everywhere by minute canals, 

 which convey the water, taken in at the pores, to large 

 canals communicating with the osteoles. At certain places 

 in the smaller canals there are enlargements, which are 

 lined with cells bearing cilia. The water currents are 

 believed to be due to the lashing action of these cilia. 



Gemmules. Imbedded in the sponge flesh, and most 

 abundant near the base of the sponge, observe a large 

 number of small, round, yellowish or brownish seedlike 

 bodies; about the size of the smallest mustard seeds. These 

 are called gemmules. 



Study gemmules which have been cleared and mounted 

 for examination. 1 Under low power of microscope, ob- 

 serve the thick, yellowish coat of the gemmule opening at 

 one side by a foraminal aperture (called, usually, simply 

 the aperture) . In the outer coat of the gemmule observe 

 spicules which differ markedly from the skeleton spicules 

 in form. These gemmule spicules vary greatly in form in 

 different fresh- water sponges. If the sponge named at the 

 beginning of this section furnished the gemmules now 

 under examination, the spicules found in their outer coat 

 will be of the form of a spool, with the discoidal ends of 

 the spool notched to form radiating points, and with the 



1 This preparation and the one following it, the student may make for 

 himself, if desired, by following directions found in Appendix, p. 280. 



