RHIZOPODA. 43 



of spun glass, is a familiar example of the flinty sponges. In 

 most cases, the skeleton, and often the flesh as well, is fur- 

 nished with more or less numerous needles or spicula, gener- 

 ally of flint, but sometimes of lime, which assume a great vari- 

 ety of shapes, and appear to exercise different functions (Fig. 

 9, c, d). 



As regards the reproductive process in the sponges, it will 

 be sufficient to state very briefly the leading phenomena which 

 have been observed in the fresh-water sponge {Spongilla 

 fluviatilis). If a specimen of Spongilla be observed toward 

 the approach of winter, its deeper portions will be found to be 

 filled with numerous small, rounded bodies, like seeds, which 

 have been called " gemmules." Each gemmule (Fig. 9, a, b) 

 exhibits at one point a small aperture, and is found to be com- 

 posed of a leathery membrane, surrounded by a layer of sar- 

 code, in which are imbedded a number of spicula. These 

 spicula consist each of a central rod or axle carrying a toothed 

 wheel or disk at each end (Fig. 9, c). In the interior of the 

 capsule thus formed is a mass of cells, of which the central 

 ones contain numerous reproductive germs. When the spring 

 comes, these masses are discharged into the water through the 

 aperture of the gemmule, and become developed into fresh 

 Spongilloe. In addition to this method of reproduction, the 

 fresh-water sponge during the summer months has the power 

 of producing true eggs or ova, and sperm-cells. The impreg- 

 nated ova develop themselves into embryos, which are pro- 

 vided with numerous cilia or vibrating hairs, by means of 

 which they swim about freely. Finally, upon finding a suit- 

 able locality, they fix themselves to some solid object, lose 

 their cilia, and grow up into SpongilloB. Indeed, as a general 

 if not universal rule, the embryos of the sponges are provided 

 with cilia, and are thus capable of active locomotion. In this 

 way is secured the extension over a wide area of these other- 

 wise fixed and plant-like organisms. 



Distribution of Sponges in Space. — It remains only to add 

 a few words on the distribution of sponges in space. With 

 the single exception of Spongilla, all known sponges are in- 

 habitants of the sea ; but the former is to be found in lakes 

 and rivers in most parts of the world. The marine sponges 

 are found mostly attached to stones and other foreign objects 

 between tide-marks and in deep water. The sponges of com- 

 merce are mostly obtained from the Grecian Archipelago, but 

 inferior kinds are imported from the Bahama Islands. One 

 common sponge {Clio?ia), instead of incrusting other objects, 



