1890-91.] TJie Structure and Life-History of a sponge. 431 



protoplasmic dot executes the delicate fairy-like sculpturing 

 of the frustules ; or in the Polycistina and the Foraniinifera, 

 where the amceboid gelatinous speck just referred to elaborates 

 the varied forms of beauty to be found among the " shells " or 

 coverings of these minute organisms. How it is done will, 

 no doubt, for ever remain one of the mysteries of life. The 

 protoplasmic mass which forms the basis of sponge structure 

 is morphologically a collection of these amceboid cells set in a 

 living matrix, each cell possessing its nucleus and granular 

 contents. The whole mesoderm or middle layer of a sponge 

 is made up of this gelatinous mass, the imbedded cells vary- 

 ing in appearance and performing different offices. Thus, 

 while some are stationary, and combine to subserve the func- 

 tions of muscular fibres or connective tissue, being connected 

 by their pseudopodia, there are others which " wander in the 

 tissue, and frequently contain large granules, looking like fat 

 or starch, serving no doubt as food reserves." Any one who 

 has collected our commoner sponges must have observed their 

 slimy appearance while growing, and the quantity of this 

 gelatinous substance wliich sometimes pours from them when 

 they are gathered. The constituent cells of this viscid fluid, 

 when seen under the microscope, behave in exactly the same 

 way as the fresh-water amreba, throwing out their pseudopodia 

 in all directions. Besides these ameboid cells of the meso- 

 derm, there are found on examination the flattened polygonal 

 pavement-cells of the ectoderm or outer layer, which cells, 

 besides, line the inhalant canals, to be presently noticed ; and 

 also the cells of the endoderm or inner layer, which line the 

 exhalant canals, and, with one exception, are of the same 

 structure as the epithelial cells of the ectoderm. 



The sponges of commerce have made every one familiar 

 with the ramifying and interlacing horny fibres which con- 

 stitute the framework of a sponge, and the numerous canals 

 and passages thus formed. But by far the gTcater number of 

 sponges have other skeletal feature.s, in the shape of mineral 

 particles, which are imbedded in the sponge-flesh and in the 

 horny fibres in varying numbers, — sometimes almost super- 

 seding the fibrous framework, as in the case of the common 

 British forms, Halichondria, Grantia, and Tethya. Some 

 sponges, again, have their framework vjJiolly composed of 



