PROTOZOA : SPONGIDA. 65 



less number of inhalant " pores." There are, in fact, many 

 who hold that the more complex sponges are merely produced 

 by the aggregation together of a number of these simpler 

 colonies. 



In a living sponge a constant circulation of water is main- 

 tained by means of an aquiferous system (fig. 8), which is 



Fi^. 8. — Diagrammatic section of SpongUla (after Huxley), a a Superficial layer or 

 'dermal membrane;" bb Inhalant apertures or "pores;" c r Ciliated cham- 

 bers; d An exhalant aperture or "osculum." The arrows indicate the direction 

 of the currents. 



constituted by the oscula and pores — already alluded to — and 

 by a system of canals excavated in the substance of the 

 sponge, and uniting the two sets of apertures. The water 

 passes in by the "pores" or inhalant apertures, and is con- 

 veyed by a series of canak — the " incurrent " or " afferent " 

 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 superfi- 

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

 chambers lined with sponge -particles which are provided 

 with vibratile filaments or cilia (fig. 8, 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 



