Structure of some Silicious Sponges. S^ 



ctdscbpical examination, on plates of glass, or between thin 

 scales of mica, where they may be preserved for any period. 



The Spongia papillaris of Pallas, Gmelin, JMontagu, and 

 Lamourou^, which is by fkr the most abundant species of this 

 animal inhabiting the British shores, affords a good illustration 

 not only of the silicious axis, but of that peculiar simple form 

 of spiculum, which I have considered as the first or most ele- 

 mentary in marine sponges. This species is mentioned by Pal- 

 las, as an inhabitant of the coast of America. Montagu foimd it 

 common on the coast of Devonshire, and I have found it abun- 

 dant on several parts of the east and west coasts of Scotland, on' 

 the north coast of Ireland, and on the shores of Bute, Islay, 

 lona, Staffa, Skye and Arran ; but I have nowhere met with it so 

 large and abundant as on the south side of the Frith of Forth, 

 where it seems to enjoy its natural climate. It is generally 

 seen at low-water mark, spreading on the sides of rocks as a 

 thick soft spongy covering, of a yellow or green colour, with 

 numerous conical tubular papillge projecting irregularly from its 

 surface ; its prominent papillae are all terminated by wide fecal 

 orifices, with thin translucent margins, sometimes quite circular, 

 and sometimes presenting a very irregular broken outline. It 

 occurs in portions, from half an incli to an inch thick, and from 

 a few inches to more than a foot in breadth ; sometimes it ap- 

 pears as a single isolated papilla, growing on the side of a rock, 

 or on the root of a fucus. Its surface is sometimes very smooth, 

 sometimes slightly corrugated and uneven, and is every where 

 covered with very minute regularly formed pores, visible to the 

 naked eye, which give the surface the appearance of the finest 

 gauze. The papillaris not only has a great range of colour 

 from bright yellow, through the intermediate shades, to dark 

 sea-green, but it likewise presents the greatest variety of surface 

 of any of the spreading species met with in the Frith of Forth ; 

 and, I have no doubt, that some of these varieties of outward 

 form, have been described and represented as distinct species. 

 The papillae, though generally very irregular in their distribu- 

 tion, not unfrequently unite from their proximity, so as to form 

 elevated ridges ; and when these ridges happen to lie in a straight 

 and parallel direction, they give the surface a crested appearance, 

 as represented by Mr EUis, (Phil. Trans, v. 55. Pi. II. Fig. G.) 



