8 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPE. 



mass of them is snowy white to the eye. Corroboration is furnished 

 by the occasional occurrence of two exceptions in colouring, viz., in 

 Ascetta coriacea and in Alcyonium digitatum. In the former case 

 the sponge frequently varies from the normal white to a lemon yellow 

 or orange, and even red, while in some rare cases Alcyonium is found 

 of an orange hue. Examination microscopically, shows in both cases 

 the new colour to lie in the ground tissue. Again, if we boil a piece 

 of calcareous sponge in potash solution and get rid thus of all the 

 animal matter, leaving spicules alone — the latter will be found to 

 compose a snowy powder at the bottom of the vessel. 



It is interesting to note that in marine animals, next to the 

 primary white, comes yellow in various shades, grading easily into 

 orange and then into red ; an interesting fact when we remember 

 that a similar colour series is made out in the acquisition of colour 

 by flowers. 



Another interesting point brought out by comparative exam- 

 ination of sponge colouring is that species with siliceous skeleton 

 spicules are well advanced in the colour scale. Scarcely any have 

 the primitive white. 



How is this ? It is not in all cases that siliceous spicules have 

 such differences in form from the spicules of calcareous sponges as 

 to lose the optical appearance of whiteness, because to take Pachy- 

 matisma as an example — even a small mass of its separated spicules 

 appears of snowy whiteness, and yet alive, the sponge is not so 

 coloured. The difference is certainly due in this case, to the optical 

 whiteness of the spicules being masked by a superior development 

 of coloured fleshy- tissue. The other siliceous sponges have also this 

 superior development of soft tissue, and this appears to have become 

 pigmented through the action of certain of the following factors : — 

 (a) a necessity for some colouring other than a dull greyness to act 

 as a " warning " colour to predatory animals, (6) the greater ease in 

 producing other pigments than white, or (c) the greater utility for 

 this purpose of such alternative colours. The former of the two 

 last mentioned is most certainly the more probable, for the white 

 calcareous sponges are quite as immune from attack as any of the 

 gorgeous scarlets and oranges of the siliceous division. What holds 

 good of these sponges applies equally to the crusting compound 

 Ascidians, which in brightness of colour rival, and at times outstrip 

 the sponges. 



