GENERAL STRUCTURE OF SPONGES. 



453 



cilia, closely resembling those of Volvox (Fig. 70, i), by the 

 agency of which, a current of water is kept up through the 

 passages and canals excavated in the substance of the mass. 



FIG. 217. 



Structure of Grautia compressa: A, portion moderately magnified, showing general arrangement 

 of triradiate spicules and intervening tissue ; B, small portion highly magnified, showing ciliated 

 cells. 



And from the observations of Mr. Carter 1 upon the early 

 development of Sponges, it appears that they begin life as soli- 

 tary Amaebce, and that it is only in the midst of aggregations 

 formed by the multiplication of these, that the characteristic 

 Sponge-structure makes its appearance, the formation of spicules 

 being the first indication of such organization. The ciliated 

 cells seem to form the walls of the canals by which the whole 

 fabric of the Sponge is traversed ; these canals, which are very 

 irregular in their distribution, may be said to commence in the 

 small pores of the surface, and to terminate in the large vents ; 

 and a current is continually entering at the former, and passing 

 forth from the latter, during the whole life of the Sponge, bring- 

 ing in alimentary particles and oxygen, and carrying out excre- 

 mentitious matter. 



296. The skeleton which gives shape and substance to the 

 mass of sarcode particles that constitutes the living animal, is 

 composed, in the Sponges with which we are most familiar, of 

 an irregular reticulation of fibres. The arrangement of these 

 may be best made out, by cutting thin slices of a piece of 

 Sponge submitted to firm compression, and viewing these slices, 

 mounted upon a dark ground, with a low magnifying power, 

 under incident light. Such sections, thus illuminated, are not 

 merely striking objects, but serve to show, very characteristically, 

 the general disposition of the larger canals and of the smaller 

 areolse with which they communicate. In the ordinary Sponge, 

 the fibrous skeleton is almost entirely destitute of spicules, the 

 absence of which, in fact, is one important condition of that 



1 " Annals of Natural History," Ser. 2, vol. iv. 



