42 THE rnodiiKssivE DEVELOPMENT 



or less elongated, finger-shaped, transparent body (s) which pro- 

 jects from one side. If this is watched attentively, even with a 

 common pocket-magnifier, it will be found to be a hollow tube ; 

 and minute particles may be seen passing in streams through 

 the tube, outwards, into the surrounding water. If now, using a 

 much higher magnifier, the surface of the sponge, at the bases 

 of the groups of bristles, be closely examined, there may be de- 

 tected from time to time the expansion of a minute aperture (/;), 

 and the passage of particles of matter inwardly. In this way a 

 constant current is kept up, from without to the interior, through 

 these numerous minute apertures ; and by a system of variously 

 united canals, the fluid and the included floating matter are 

 transmitted through the body of the sponge and poured into one 

 main channel, and from there they pass into the projecting trans- 

 parent conduit (s), and finally make their exit from its terminal 

 aperture. By this process the sponge obtains its food. The 

 circulation, which is at times fitful, is produced by the vibration 

 of minute cilia which cover the interior of the anastomozing 

 tubes, and by the occasional contraction of the whole body. In 

 the latter case the particles of loose matter within the canals are 

 ejected with considerable violence. The body consists of a soft, 

 highly extensile and contractile, transparent, filmy substance, in 

 which groups of bristles (spicula?), of a horny nature, are im- 

 bedded at irregular intervals, and through which the channels, 

 which I have mentioned, run in every direction. The color is 

 due to numerous granules which lie within its substance ; other- 

 wise it would be perfectly glassy in hue ; as it really is at the 

 surface, and in the digitiform emptying conduit. Although I 

 have applied the best and highest powers of the microscope to 

 the most transparent and clearest portions of the body, and to the 

 conduit (s), I have not been able to make sure that there is a 

 cellular structure in the soft tissue. It is true that occasionally 

 I have detected in the conduit what appeared to be a decided 

 cellular tissue, consisting of distinctly nucleated, closely packed, 

 polygonal cells, but at other times I could not see the least trace 

 of such ; nor have other observers been more fortunate than my- 



