254 EDWARD A. MINCHIN. 
ing at the same time to thoroughly work out the anatomy and 
histology of this and other Plymouth species of Leucosolenia. 
As, however, I was obliged to leave England for Naples at an 
early stage of my investigations, I thought it best to publish 
an account of this membrane at once, together with a few 
scattered observations on the histology of the sponge, hoping 
at some future time to make a more complete study of this in- 
teresting sponge genus. 
In a typical osculum the interior of the chimney-like tube is 
seen in sections to be lined by a layer of collared epithelium, 
only interrupted at intervals by the openings of pores (fig. 1). 
At a certain height the layer of the collar cells stops abruptly, 
but the wall of the oscular tube is continued on for a short 
distance as a funnel-like expansion (“ risselformige Mund- 
Offnung ”’) consisting of jelly containing spicules and lined by 
ectoderm. Immediately above the layer of collar cells the 
sieve membrane stretches across the opening. It is thus some 
distance below the actual margin of the oscular opening. Figs. 
1, 2, 3, 12 a and 4, and 18 show the membrane in section. 
Figs. 6 and 9 represent portions of it macerated out in gly- 
cerine, the portion in fig. 1 having been previously treated with 
weak acetic, and that in fig. 9 stained in picrocarmine. Fig. 
5 shows a side view of a whole osculum (the one from which 
fig. 6 was dissected out) mounted in glycerine after fixation 
with osmic and removal of the spicules by dilute acetic. Fig. 
10 shows a view from above of a whole osculum mounted in 
Canada balsam, after having been fixed with absolute alcohol 
and stained in hematoxylin, while 10 a represents the entire 
sieve membrane of the same osculum, drawn with a somewhat 
low magnification. Finally, fig. 11 @ and 6 represent two 
consecutive sections from a series taken across an osculum trans- 
versely but slightly obliquely, so that portions of the membrane 
are obtained flat. 
The sieve membrane varies in size, naturally, with the dia- 
meter of the osculum. The smallest open osculum I have 
seen was about 175 u in diameter, the largest about 465 yu, or 
nearly half a millimetre. The osculum without any opening 
