SILICEOUS AND HORNY SPONGES 87 



communicate with the exhalant canals by narrow canals called 

 aphodae, as in the majority of the siliceous sponges. When the 

 sponge is prepared for human use, the soft parts are allowed to die 

 and rot, leaving the horny skeleton, which is then cleaned. The 

 large holes on the upper part of the dried skeleton mark the 

 position of the oscula ; in its interstices formerly lay the ramifica- 

 tions of the canal system. The softness and wearing qualities of the 

 sponge depend upon the fineness of the meshwork of its skeleton, 



Fig. 55. — A diagram of the structure of a bath sponge. 



exh.c, Exhalant canal ; inh.c, inhalant canal ; fl.c, flagellated chamber ; osc, osculum ; ost.. ostium ; 

 sd.c, subdermal cavity ; sk., one of the principal pillars of the skeleton, containing embedded sand 

 grains ; sk'., minor fibres of the skeleton. 



and upon the amount of the sand particles which are embedded 

 in it. The true Bath Sponge {Euspongia officinalis) has very few 

 foreign particles. It is gathered in 10 to 15 fathoms of water, the 

 finest varieties from the Adriatic, coarser ones from elsewhere in 

 the Mediterranean, the West Indies, and Australian seas. Various 

 species of Hippospongia yield coarse kinds of sponge. 



Sponges have free larvae, of several different kinds, but all are 

 covered with flagellate cells, by which they swim. The remarkable 

 feature of the metamorphoses by which these larvae become the 

 fixed adults is that the flagellated cells pass into the interior, 

 develop coUars, and become the choanocytes. 



