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 water enters on all sides. These presently 

 unite into slender pipes, which, irregularly meandering, 

 are continually uniting into larger and yet larger canals ; 

 of which 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 vol- 

 canoes, 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 re- 

 mainder 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 we use 

 on our dressing-tables ; each consisting of a cylindrical 

 slender rod, pointed at one end, and at the other sur- 

 mounted 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, which seem to be entangled together 

 quite at random without rule or arrangement. And yet 

 there is an arrangement discernible here ; for the canals 

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