39 



skeleton, it will be found that throughout the numerous 

 orders, families and species of sponges, an essentially 

 similar structural plan is associated with their living 

 elements or so-called flesh. The familiar fresh-water 

 sponge {Spongillafluviatilis), already referred to as abound- 

 ing in the upper reaches of the Thames, may be selected 

 as a suitable type for illustrating the more important of 

 these structural points. On withdrawing a living mass of 

 this sponge from the water, it will be found that a very 

 large portion of the vital elements, will drain away in the 

 form of a transparent slime or mucous. If, in place of 

 leaving it to drain, we immediately detach a fragment or 

 place the whole mass in a glass of water, the following 

 points and phenomena may be observed. A delicate semi- 

 transparent, film-like structure, having much the aspect of 

 a closely-woven cobweb, will be seen standing out from 

 the more solid central substance of the sponge ; this 

 structure at one or more points is developed outwards in 

 the form of a conical or cylindrical tube, and is the ex- 

 current aperture or so-called "flue or osculum," out of 

 which, on attentive examination, minute particles of refuse 

 food matter may be seen issuing after the manner of smoke 

 from a chimney-top. A yet closer inspection, with the aid 

 of a pocket-lens or low-power of the microscope, will reveal 

 the fact that an almost indefinite number of very minute 

 circular openings or perforations pierce the substance of 

 the delicate superficial film on every side, and that at these 

 small apertures currents of water carrying the minute 

 organic particles upon which the sponge animals feed, are 

 more or less constantly flowing in ; these are the incurrent 

 apertures or so-called "pores." To arrive at a correct 

 comprehension of the internal structure of the sponge, and 

 of the motive force of the incurrent and excurrent streams 



