22 



PROTOZOA. 



Fig. 10. 



sarily abound in the water of the ocean, are thus introduced into the 

 sponge on all sides, and are probably employed as nutriment, whilst 

 the superfluous or effete matter is continually cast out with the issuing 

 streams as they rush through 

 the fecal orifices. The growth 

 of the sponge is thus provided 

 for ; the living gelatinous por- 

 tion continually accumulates 

 and, as it spreads in every di- 

 rection, secretes and deposits, 

 in the form peculiar to its spe- 

 cies, the fibrous material and 

 earthy spicula constituting the 

 skeleton. 



(50.) It is by no means easy 

 to explain the cause of the per- 

 petual flow of water through 

 the substance of the sponge in 

 currents so powerful and so 

 constant. In the various spe- 

 cies of Grantia, however, Mr. 

 Bowerbank and Dr. Dobie have 

 succeeded in detecting the pre- 

 sence of cilia. These sponges 



\ 



A, a common Sponge : the arrows indicate the di- 

 rection of the entrant and issuing currents. B, a 

 ciliated gemmule magnified. 



have a very simple structure, each being a sort of bag, whose walls are 

 so thin that no system of canals is required, the water absorbed by the 

 outer surface passing directly towards the inner, and being expelled from 

 the mouth of the bag. The cilia may be plainly seen with a -J-th inch 

 objective on the cells of the gelatinous substance scraped from the 

 interior of the bag, or they may be observed in situ by making very thin 

 sections of the substance of the sponge. 



Mr. Bowerbank*, however, has satisfactorily proved that some sponges 

 possess a power of opening and closing the oscula at pleasure. He found 

 that in a specimen of Spongilla fluviatilis about half an inch in dia- 

 meter, which had attached itself to a watch-glass, there was at the 

 summit of a large oval inflation a single osculum, which opened or 

 closed according to the necessities of the animal, and from which, when 

 in full action, a constant stream of water was poured forth. The inha- 

 lation of the water by the porous system presented some remarkable 

 peculiarities : when in a state of repose, the dermal membrane appeared 

 to be completely imperforate ; but when about to commence vigorous 

 inhalant action, a slight perforation appeared here and there over its 

 surface, the orifices gradually increased in size until the full diameter 

 of the pores was attained, and their margins then became thickened and 

 * Quarterly Journal of Microscopical Science, vol. vi. p. 78. 



