RHIZOPODA. 



41 



masses of sarcode, all more or less completely independent 

 of each other, and each very closely resembling an Amoeba. 

 These separate " sponge-particles," or " sarcoids," as they are 

 called, consist, in fact, of granular sarcode, capable of pushing 

 out little processes or threads of sarcode in the form of pseudo- 

 podia, and sometimes furnished with an internal solid mass or 

 nucleus (Fig. 3, c). In some cases each sarcoid carries a 

 single lash-like vibrating filament or cilium (Fig. 3, <#, e). 

 Each sarcoid has the power, if detached, of independent move- 

 ment, and each can obtain food for itself. As the sponge, 

 however, is a fixed animal, some provision is necessary by 

 which food shall be conveyed to the sarcoids in the interior of 

 the mass. This is effected by a remarkable water-carrying or 

 "aquiferous" system in the following manner: The entire 

 sponge is riddled in every direction by an immense number 

 of canals, all opening on the surface, and communicating freely 



FIG. 8. Diagrammatic section of Spongilla (after Huxley), a a Outer or superficial layer 

 of the sponge ; b b Inhalant apertures, or " pores ; " c c Ciliated chambers ; d An exha- 

 lant aperture, or " osculurn." The arrows indicate the direction of the currents. 



with one another in the interior of the mass. The canals are 

 of different sizes, and, as can readily be observed in an ordinary 

 sponge, their external openings are also of different sizes. A 

 few of the holes are of much larger size than the others, and 

 these, for reasons which will be seen directly, are called the 

 " exhalant apertures," or " oscula." The great majority of the 

 holes are very minute, and these are known as the " inhalant 

 apertures," or " pores." In a living sponge a more or less 

 constant circulation of water is carried on by means of this 

 canal system. The water is admitted by means of the pores 

 (Fig. 8, b #), is driven into the interior of the sponge, and is 

 finally expelled in steady streams from the oscula (Fig. 8, d). 

 The mechanism by which this circulation of water is effected, 



