AlfD PHYSIOLOGY OF THE SPONGIAD^. 
759 
Prehensile Fibre. 
In the course of my examination of the fibrous skeleton-tissues, I have found but one 
instance in which they have developed prehensile organs to assist in the attachment of the 
sponge, and this is in a minute siliceo-fibrous species, parasitical on the base of a specimen 
of Oculina rosea., from the South Sea. In this sponge the basal fibres curve downward in 
the form of numerous small, nearly semicircular reversed arches, from the lowest portions 
of each of which there is a short stout portion of fibre projected ; and at about the length 
of its own diameter downwards, a ring of stout prominent bosses, six or eight in number, 
is produced, very considerably increasing its diameter at that part, immediately beneath 
which the fibre is attenuated to a point. These singular organs are admirably calculated 
to penetrate the porous cawties or fieshy envelopes of the coral, and thus to securely 
attach the sponge to its adopted matrix (Plate XXVIII. fig. 12). 
Cellular Tissue. 
The cellular structures in the Spongiadse are few and very simple in form. We find 
no series of conjoined cells in the body of the sponge, as in vegetable tissues. The 
only forms in which true cellular structures occur in the bodies of sponges, are those of 
detached spherical molecular cells, and of discoid or lenticular nucleated cells. Cellular 
structures of the first form are found in abundance on the fibres of many species of the 
true sponges, and are believed by Dr. Johnston to be the reproductive organs of that 
genus. They are very minute ; an average-sized one measured inch in diameter. 
They are pellucid, and afford no indications of a nucleus, either single or multigranulate. 
Imbedded in the sarcodous stratum on the interstitial membranes in many of the 
Halichondroid tribes of sponges, we frequently find numerous compressed circular cells. 
In the greater number of cases they are so translucent as to readily escape observation, 
even with a tolerably high power ; but in other species, as in Fdonemia acervus, Bower- 
bank, MS., a new genus of sponges from the South Seas, in the collection of the Eoyal 
College of Surgeons, and in Halichondria nigricans., Bowerbank, MS., a British species, 
these tissues are developed in a more than usually distinct condition. 
In the first-named sponge they are thickly dispersed on the surfaces of the interstitial 
membranes, but without any approach to order or arrangement. They are decidedly 
lenticular in form, with a well-defined transparent nucleus, which varied in size from 
about one-fourth to three-fourths the diameter of the cell in which it was contained. 
The cells varied considerably in size ; the largest I could find was 3 x 5 ^ inch in diameter, 
and one of the smallest tohwo inch, but the greater number were about inch in 
diameter (PL XXVIII. fig. 13), In Halichondria nigricans they do not appear to be 
quite so convex as in Fdonemia acervus, nor are they so numerous as in that species, but 
they are somewhat larger in size ; one of the largest measured ysVc) inch in diameter, 
and a small one 5 -^ 5 ^ inch ; they are represented m situ in Plate XXVIII. fig. 14. 
The only instance with which I am acquainted of a conjoined arrangement of such 
