PORIFEBA OR SPONGES. 141 



work of horny fibres, strengthened by spicules of mineral 

 matter, sometimes calcareous, but more commonly siliceous. 

 The entire mass is traversed by a great number of canals, 

 which may be said to commence in the small pores upon its 

 surface, and which discharge themselves into the wide canals 

 that terminate in the large orifices, or vents, that usually pro- 

 ject more or less from the surface 

 of the Sponge. Through this sys- 

 tem of canals, there is continually 

 taking place, during the living state 

 of the animal, a circulation of water, 

 which is drawn in from without 

 through the minute pores, then 

 passes into the large canals, and is pig gl _ SPONQE 



ejected in a constant stream from 



the vents. The immediate cause of this movement seems 

 to lie in the vibration of cilia so extremely minute that their 

 existence can only be detected by the most careful micro- 

 scopic examination. Its purpose is evidently to convey to 

 the animal the nutriment which it requires, and to carry oif 

 the matter which it has to reject. No distinct indications of 

 sensation, or of power of locomotion, have been seen in the 

 Sponge : but changes in the form of its projecting vents 

 may be seen to take place from time to time, if it be watched 

 sufficiently long. 



1 37. The reproduction of the Sponge is commonly effected 

 by the budding forth of little particles of sarcode, from the 

 layer which lines the larger canals ; these become furnished 

 with cilia, and, when detached and carried out by the current 

 that issues from the vents, swim freely about for some time ; 

 so as, before fixing themselves and beginning to develope 

 into Sponges, to spread the race through distant localities. 

 But it appears that Sponges are also reproduced by a true 

 sexual process ; " sperm-cells " and " germ-cells " being pro- 

 duced (as in the Hydra, 123) in different parts of the 

 organism, and a true embryo taking its origin in the action 

 of the contents of the former upon those of the latter. 



