THE SPOXGIDA OR PORIFERA. 349 



finely-powdered indigo to the water. The particles of indigo could 

 be seen passing in great numbers through the pores, coursing 

 along in different directions through the sponge, and many of the 

 larger particles rushing with excrementitious granules through the 

 oscula. The smaller particles of indigo were found to be mostly 

 retained in the sponge, so that, if the supply of indigo was main- 

 tained for an hour, the whole sponge became of a distinctly blue 

 colour, regaining its usual pellucid aspect in about twelve to 

 eighteen hours subsequently, from the digestion in its interior of 

 the particles of indigo, and the ejection through the oscula of all 

 excrementitious matter. 



It was also noticed by Bowerbank that the water-currents in 

 the Spongida varied considerably, at different periods, in rapidity, 

 volume, and force. However quiescent a sponge might seem, he 

 usually observed a gentle and continued circulatory action, though 

 the larger oscula were lessened in size, and the smaller ones, toge- 

 ther with multitudes of the pores^ quite closed. This gentle cir- 

 culation is sufficient for the oxygenation of the cells and soft 

 gelatinous sarcode^ which form the living portion of the sponge, 

 sufficient, in a word, for the respiration of the animal. Y ox feeding 

 purposes, Bowerbank showed that all \\\q pores were opened widely, 

 the oscula increased in dimensions, the water-currents rapid, forc- 

 ible, and full, innumerable particles of food being swept through 

 \\\^ pores into the interior of the sponge, where they adhered to 

 the sticky, sarcodous lining of the many passages, and became 

 digested, the excrementitious and collapsed debris being finally 

 ejected through the oscula. 



Structure of Sponges. 



When we come to the structure of Sponges, we enter on 

 debateable ground. In a general way, it may be stated a sponge 

 consists of a mass of transparent or semi-transparent, gelatinous 

 or treacly, sticky and tenacious substance, called sarcode, permeated 

 in various directions by the channels of a canal system, which is 

 lined in a greater or less degree by ciliated and collared cells, and 

 supported by a skeleton, consisting either of calcareous, siliceous 

 (or flinty), or keratose (or horny) spicules or fibres. The entrance 

 to, and the exit from, the canal system are both, as has been 



