156 The Commercial Products of the Sea. 



tinous film, which, being provided with cilia, causes a 

 current of water to pass in at the smaller pores and out at 

 the larger apertures, the sponge probably assimilating the 

 nutritive particles which enter into the water. 



Fig. 4. 



Sponge showing the outgoing water currents. 



A monograph of these polypes, published in the 20th 

 volume of " Des Annales du Museum, Paris," enumerated 

 141 species, ranged under six divisions. 



The sponges of commerce are divided into two scientific 

 divisions : — 



1. Comprehending the common sponges (Spongia 

 officinalis), of rounded or flat form, convex beneath, of soft 

 tissue, more or less tenacious, large pored with great 

 orifices. 



2. The second division includes the fine sponges 

 (Spongia usitatissimuni), of concave or cup-like form, 

 having the oscules slender like hair, and the pores very fine 

 in the interior. Of these there are 34 species. 



