42 



INVERTEBRATE ANIMALS. 



was long unknown, but it is now known to consist in aggrega- 

 tions of sponge-particles provided with cilia which all work 

 toward the interior of the sponge (Fig. 8, c c). The circula- 

 tion of water in this manner can be readily observed in many 

 of our common marine sponges, and it is under the control of 

 the animal to a great extent. The large apertures or oscula 

 are permanent, but they can be closed and opened at will ; 

 while the smaller apertures or pores appear to be formed 

 afresh, wherever they are wanted, at any point of the surface. 

 By means of the currents of water each individual sarcoid or 

 sponge-particle is enabled to obtain food, so that the whole 

 sponge, as remarked by Huxley, " represents a kind of sub- 

 aqueous city, where the people are arranged about the streets 

 and roads in such a manner that each can easily appropriate 

 his food from the water as it passes along." It is also not 

 improbable that the process is at the same time a rudimentary 

 form of respiration. 



4*10. 9. ft Gemmule of Spongilla\ 7t Hilum; & Diagrammatic section of the gemmule* 

 showing the outer layer of spicules or amphidiscs, and the inner mass of cells ; h Hilum ; 

 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 magni- 

 fied. 



Such, then, are the general phenomena exhibited by any 

 sponge, and the chief variations which occur among the 

 sponges are to be found in the nature of the skeleton. In the 

 sponges of commerce the skeleton consists of matted fibres 

 composed of a substance nearly allied to horn. In other forms 

 the skeleton is calcareous, or composed of lime; and in other 

 cases, again, it is siliceous, or composed of flint. The Venus's 

 flower-basket (Euplectella), which looks like a goblet woven 



