A SIEVE-LIKE MEMBRANE IN LEUCOSOLENIA. 251 
Note on a Sieve-like Membrane across the 
Oscula of a Species of Leucosolenia, with some 
Observations on the Histology of the Sponge. 
By 
Edward A. Minchin, 
Assistant to the Linacre Professor of Human and Comparative Anatomy in 
the University of Oxford. 
With Plates X and XI. 
At Plymouth, on the rocks immediately beneath the Marine 
Biological Laboratory, there are to be found at low tide a great 
number of calcareous and other sponges. Among them occur at 
least two quite distinct species of Leucosolenia, which can even 
be distinguished at sight by their mode of growth. The first 
species, which has only triradiate spicules (genus Ascetta, 
Haeckel), forms a network of anastomosing tubes, which at first 
creep close round the seaweeds and other objects, but finally, 
in large specimens, form great white masses of as much as two 
inches or more in height. From the network of tubes arise here 
and there the chimney-like oscula, which are simply continua- 
tions of the tubes, and not marked out by their greater diameter 
from them. The diameter of both the oscula and the ordinary 
tubes may vary within limits, but the diameter of the oscula is, 
if anything, less (fig. 10), at any rate not markedly.greater than 
that of the tubes. On the other hand, in the second species 
of Leucosolenia, which has tri- and quadri-radiate spicules 
(genus Ascaltis, Haeckel), the mode of growth is essentially 
the same, but the oscular tubes are at once marked off from 
the remainder of the sponge by their very much larger size 
