SPONGES 



477 



rfd 



water is proceeding out of the sponge, and that the latter must accord- 

 ingly be endowed with life. 



(b) As in lamellibranchs (see p. 437), this current is produced by the 

 constant movements of cilia which are developed on the walls of small 

 cavities, the so-called "flagellated chambers." The water is carried 

 to these chambers by numerous canals (afferent canals), which open by 

 numerous small pores on the surface of the sponge. Other (efferent) 

 canals conduct the water from the flagellated chambers to a larger 

 channel, from which it 



is finally expelled by 

 one of the large surface 

 openings above men- 

 tioned, which are known 

 as oscula. 



(c) As in the lamel- 

 libranch mollusca (see 

 p. 438), the respiratory 

 current constantly con- 

 veys to the sponge fresh 

 water for respiratory pur- 

 poses, and at the same 

 time food. The latter, 

 as in the bivalve mol- 

 luscs, can only consist of 

 the minutest living or- 

 ganisms and of finely- 

 divided, decaying mat- 

 ters contained in the 

 water; for a sponge, in 

 the absence of prehensile 

 organs, such as we have 



DIAGRAMMATIC REPRESENTATION OF THE STRUCTURE OF A 

 SPONGE. 



P., Pores on surface; K., canals by which the water is 

 carried to the flagellated chambers (G.), and thence to the 

 cavity (H.) ; A., osculum or exhalant aperture, 

 denote the direction of the current. 



The arrows 



seen to exist in its nearest 

 allies, the likewise sessile 

 polyp, would be unable to capture prey of larger size. 



(d) No specialized organs of respiration and digestion are developed 

 in sponges, any more than in coelenterates. There is no single body 

 cavity, but as we have seen in Section C, &, numerous canals and 

 chambers. In many respects the sponges differ in structure from all 

 other animals. 



In the sponges which we have discovered we notice, however, several 

 exhalant apertures, or oscula, to which, accordingly, several branched 

 canal systems must correspond. Such is evidently the case, but only in 



