PROTOZOA AND SPONGES. 387 



blank spaces which are larger towards the centre, smaller 

 and more numerous as they approach the exterior. 



These openings are sections of so many canals, by 

 which the whole substance of a sponge is permeated. 

 The surface is perforated with minute pores, at which the 

 surrounding w^ater enters on all sides. These presently 

 unite into slender pipes, which, irregularly meandering, 

 are continually uniting into larger and yet larger canals ; 

 of w T hich the greater open spaces that you see are the 

 oblique divisions. These have certain outlets, called 

 oscula, on the surface, from which the stream is poured 

 that has thus made the grand tour of the whole interior. 

 Such oscula, as you perceive on the remainder of the 

 Halichondria, are usually raised on slight eminences ; 

 and resemble, especially when in living action, miniature 

 volcanoes, vomiting torrents of water and granules of 

 effete matter, instead of fire and ashes. 



During life these granules were much more diffused, 

 and formed a considerable portion of the living flesh, 

 the remainder being composed of a glairy sarcode, almost 

 fluid. The whole was maintained in position by the 

 solid spicula of flint, which you see abundantly in this 

 slice. These take a curious form, exactly that of the 

 pins which w T e use on our dressing-tables ; each consist- 

 ing of a cylindrical slender rod, pointed at one end, 

 and at the other surmounted by a globular head, the 

 whole formed of glass, — -flint glass literally. You see 

 them bristling all round the edge of the section, being 

 stuck into the surface of the sponge, exactly as pins are 

 loosely stuck into a pin-cushion. The heads and points, 

 too, project into the cavities; more, however, than they 

 did during life, for you must make allowance for the 

 shrinking of the soft parts ; and thus you perceive how 

 the whole structure is permeated by these glassy pins, 

 Avhich seem to be entangled together quite at random 

 without rule or arrangement. And yet there is an 



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