THE BRITISH SPONGES. 17 



is very common to find growing on the same rock or sea- 

 weed, a siliceous, a calcareous, and a horny sponge ; they 

 have all the same exposure, and are all recipients of the 

 same nutriment, yet does each act upon this differently. 

 One extracts from the fluid silica, which it causes to as- 

 sume a solid crystalline form ; another selects in the same 

 manner the calcareous particles, which, obedient to the 

 laws of life, assume figures novel to them in their mineral 

 state ; and again, another rejects both the lime and flint as 

 injurious to its constitution. 



The propagation of sponges is another disputed part of 

 their economy. That they are not born of love is very cer- 

 tain, for no one, since Pliny's time, has ever imagined that 

 there was any sexual difference in the individuals of any 

 species. The fresh-water sponge produces abundantly, 

 when in favourable situations, oviform bodies or capsules 

 filled with germinal grains. On the bursting of the cap- 

 sules, these are set free, and by a locomotion, probably de- 

 pendant on extrinsic causes, scatter themselves abroad, and 

 diflFiise the species. (Fig. 7). The capsules are, on the con- 



Fig.1. 



trary, unmoveable, and often germinate without bursting ; 

 while also many specimens of the sponge never produce 

 them, but propagate their kind by seminal granules alone, 

 which' appear to be merely detached particles of the orga- 



