SPONGES. 269 



The canals are lined with a soft gelatinous animal 

 matter, up to the opening of the pores themselves. 

 The pores are strengthened, and probably kept 

 open, by curious little needle-like bodies, called 

 spicula, which are either siliceous or calcareous. 

 Whilst the animal is alive, the water entering into 

 the sponge by the pores circulates in the canals of 

 the sponge, and is finally expelled through the 

 larger openings, called orifices (or oscula), which are 

 also observable on the surface, interspersed among 

 the pores. 



The currents thus observed are generated 

 either by a ciliary apparatus existing in the 

 gelatinous substance which lines the canals, or by 

 capillarity.* 



The currents from the orifices are best observed 

 by placing a sponge, whilst alive, in a shallow dish of 

 water, upon which a little powdered chalk has been 

 thrown. The motions of the atoms of chalk will 

 indicate precisely the direction of the currents. If 

 the gelatinous matter which lines the canals be 

 separated, by hot water, from the tissue or skeleton, 

 the latter may be then examined under the micro- 

 scope. 



The gelatinous substance putrifies easily ; it is of 

 various colours, but principally yellowish-brown, and 

 resembles the soft part of polypes. 



* Consult on this Dutrochet, in the Memoirs cited on p. 271. 



