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THK MECHANICAL FUNCTIONS. 



The material of wliich the fleshy portion is com- 

 posed is of so tender and gelatinous a nature that the 

 slightest pressure is sufficient to tear it asunder, and 

 allow the fluid parts to escape ; and the whole soon 

 melts away into a thin gelatinous liquid. When ex- 

 amined with the microscope, the soft flesh is seen 

 to contain a great number of minute grains, dis- 

 seminated through a transparent jelly. Every part 

 of the surface of a living sponge (as may be seen 

 in Fig. 53) presents to the eye two kinds of ori- 

 lices; the larger, having a rounded shape, and 

 generally raised margins, which form projecting 



55 



papillae ; the smaller, being much more numerous, 

 and exceedingly minute, and constituting what 

 are termed the pores of the sponge. 



It was, for a long time, the received opinion 

 among naturalists that this superficial layer of gela- 

 tinous substance is endowed with a considerable 

 power of contractility : it was generally believed 

 that it shrunk from the touch, and that visible tre- 

 mulous motions could be excited in it by punctures 

 with sharp instruments, or other modes of irritation. 

 These notions are of very ancient date, for they 

 may be traced even beyond the time of Aristotle ; 

 and they have been handed down by succeeding 



