72 MANUAL OF ZOOLOGY. 



canals to a second series of tubes the " excurrent " or 

 " efferent " canals by which it reaches the " oscula " and is 

 finally expelled from the body. These processes are regularly 

 performed, and their mechanism was long a subject of specu- 

 lation. It is now known, however, that beneath the super- 

 ficial layer or " dermal membrane " of the sponge there exist 

 chambers lined with sponge-particles which are provided 

 with vibratile filaments or cilia (fig. 1 2, c, c). The pores open 

 into these chambers, and from them proceed the incurrent 

 canals, each being dilated at its commencement into a sac, 

 which is also lined with ciliated sponge-particles. By the 

 vibratile action of these cilia, currents of water are caused to 

 set in by the pores ; and as out-going currents proceed from 

 the oscula, a constant circulation of fresh water is maintained 

 through the entire sponge. In this way each individual 

 sponge-particle is enabled to obtain nutriment : the process 

 being at the same time not improbably a rudimentary form of 

 respiration. The chambers or sacs lined with ciliated sarcoids 

 have been shown by Mr Carter to be the essential element in 

 the organisation of the fresh-water and marine sponges, and to 

 be the fundamental expression of the alimentary system. 



The reproduction of sponges may be effected either 

 asexually or sexually, the following being a brief outline of 



J^g- 13- a Gemmule of Spo ngilla : h Hilum ; b Diagrammatic section of the gem- 

 mule, showing the outer layer of amphidiscs and the inner mass of cells ; c One 

 of the amphidiscs seen in profile ; d Fragment of the skeleton of a horny sponge 

 (after Bowerbank), showing the interlacing horny fibres with spicula. All much 

 magnified. 



the phenomena which have been observed in the common 

 fresh-water sponge (Spongilla), in which the process has been 

 most accurately noticed. 



