10 THE BRITrSH SPONGES. 



after death, they pass througli a bluish colour to black, by 

 putrefaction. The whole body of a sponge is specifically 

 heavier than sea-water ; and each of its parts taken sepa- 

 rately sinks in that element."* A very few of a green co- 

 lovu' inhabit our ponds and sluggish rivers ; the rest are 

 marine. Of these many of the calcareous and siliceous 

 kinds inhabit the shores between tide-marks, preferring a 

 site near the low ebb, where, nevertheless, they are daily 

 alternately submerged and left exposed to the atmosphere. 

 The figured sponges with a fibrous texture, to M^hatever 

 genus they belong, and the fleshy Tethya, are the denizens 

 of deeper water, and are never left uncovered. They 

 grow, usually in groups, on rocks, shells, shell-fish, coral- 

 lines and sea-weeds, and either have no power of selection, 

 or the quality of the site is indifferent to them. When, how- 

 ever, from the locality, they are exposed to a greater than 

 usual agitation of the sea, it is said that the structure of 

 the sponge becomes denser and more compact than in its 

 normal condition of developement. 



In their growth some sponges assume a determinate 

 figure, or at least one whose variations are confined within 

 certain limits that do not render the form nugatory as a 

 specific character; but the greater number are very ir- 

 regular and variable, their shape depending, in a great 

 measure, on the peculiarities of their site, to which they 

 easily accommodate themselves. Thus they will incrust 

 a shell or a crab, a rock or sea-weed, following every pro- 

 tuberance and sinuosity ; and the offshoots will spring up, 

 with a more luxuriant growth, in the deeper sheltered 

 places, until they render the original shape of the thing they 



* Editi. Phil. Journ. xiii. p- 96. 



