86 



MANUAL OF ZOOLOGY. 



however, in countless numbers, are spherical groups of flagellate 

 sarcoids, those of each group so arranged as to enclose a central 

 space, into which water is admitted in a manner to be subse- 

 quently alluded to. The sarcoids are so disposed that the flagella 

 of all are directed jnwards jnjto tfte_central space, and each has 

 its flagellum surrounded by a membranous collar, within which 

 the lash can be retracted when .not in use (fig. 24, B). Each 

 sarcoid can nourish itself, and can discharge the indigestible 

 portions of its food; and the surrounding water is admitted 

 into, or shut out from, the chamber formed by the sarcoids in 

 accordance with the temporary needs of the colony. These 

 spherical communities of flagellate sarcoids constitute the 

 so-called " ciliated chambers," and are to be regarded as the 

 essential elements of the sponge. Lastly, as we have seen, 

 portions of the sponge-flesh cannot be resolved in this way 

 into separate sarcoids, but the latter have apparently coalesced 

 so as to form a continuous and seemingly structureless " syn- 

 cytium." This change is especially liable to take place in the 

 layer of sarcode (" dermal membrane " or " ectoderm ") which 

 covers the exterior of a living sponge. 



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

 tained by means of an aquiferous system (fig. 25), which is 



Fig. 25. Diagrammatic section of Spongilla (after Huxley), a a Superficial layer or 

 "dermal membrane;" bb Inhalant apertures or "pores;" c c Ciliated chambers; 

 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 canals the " incurrent " or " afferent " 

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



