SPONGES 



25 



Green is a rare colour amongst marine sponges, but is the usual 

 tint of the fresh-water SpongiUinae, where, however, it is due to 

 chlorophyll. When the chlorophyll is not developed, fresh-water 

 sponges are usually brownish. In marine forms chlorophyll is 

 seldom, if evei', found as a pigment, and the nearest approach to 

 the bright green of Spongilla is a dull olive-green of not infrequent 

 occurrence. 



Although Calcarea are usually colourless, some remarkable and 

 instructive instances to the contrary are found amongst them, especially 

 in certain species of Clathrina. Thus CI. coriacea, common along the 

 shores of the British Channel, has a wide range of colour variations, 

 dift'erent specimens being white, yellow, orange, red, or lilac. The 

 particular colour which a colony assumes does not seem to bear any fixed 



i'lr.. 40. 



Aplysina acroplwha, Ndo. (after 

 , Schulze). %. 



Via. 41. 



Oscarella lobularis, O.S. (after Schulze). 

 Natural size. 



relation to other characters of its form or structure, nor is it as a rule cor- 

 related with its habitat, since specimens of the most diverse hues may be 

 found in the closest proximity, growing even on the same stone. On the 

 other hand, the specimens of this species living below the ordinary tide- 

 marks in certain localities are constantly of a pale lemon-yellow colour, 

 and this tint has become fixed as the constant colour of an allied species, 

 CI. clathrus, of the Mediterranean, while CI. ^irimordialis, another 

 Mediterranean species, shows the same variability as CI. coriacea. The 

 larvae of each colour variety have the same tint as their parent, but it is 

 not certain how far the colour is constant during the life-history of a 

 given individual. It is not improbable that it may change according to 

 the circumstances of its metabolism or from other causes at present unknown, 

 since the peculiar cell-granules, which are the seat of the colour, are very 

 variable in cpiantity and may be almost entirely wanting (temporarily ?) 

 in some specimens. 



The colouring-matter of sponges is contained in cells of the 

 dermal layer, especially in the epithelium as a rule. Special jjigment 

 cells are not found. The colouring-matter is usually very fugitive 



