1104 
DE. J. S. BOWEEBANK ON THE ANATOMY 
extremity of the sponge, — the essential difference being that in Grantia the distal end 
of the cloaca is open, and in Alcyoncellum it is partially closed by a cribriform veil, the 
orifices of which appear to be the true oscula of the sponge. And this opinion is justi- 
fied by the structure of the numerous cloacae in the closely-allied genus Polymastia. 
where we find the orifices through which the incurrent streams are poured into the 
cloaca permanently open. 
All the known species of this genus appear to consist of a single fistulose body, and 
some of them are apparently of a parasitical habit. Alcyoncellum aspergillum especially 
is fm’nished with numerous recurvo-quaternate spicula at its base, by which it attaches 
itself to sponges or other bodies. These prehensile organs do not appear in all the 
species of the genus ; and in one perfect and beautiful specimen in the Museum of the 
Jardin des Plantes at Paris the base is closed, and is entirely destitute of prehensile 
spicula. The attachment of the sponge is partly, on one side, in the form of a thick 
incrustation, and partly, close to the base, by a similar patch of thickened tissue. There 
is also another striking difference in its structure ; and that is, the absence of the raised 
margin to the oscular area at the apex of the sponge. In other structural characters it 
agrees exceedingly closely with A. aspergillum. 
Polymastia, Bowerbank. 
Skeleton a basal mass ; central portion consisting of a plexus of contorted anastomosing 
fasciculi, resolving themselves near the surface into short straight bundles disposed 
at nearly right angles to the surface. Oscula congregated, elevated on numerous 
long fistulse. Fistulse composed of numerous parallel fasciculi, radiating from the 
base to the apex of each in straight or slightly spiral lines. Plate LXXIII. fig. 2. 
This genus is closely allied to Alcyoncellum., Quoy et Gaimard, the principal differ- 
ence being that in the latter the sponge always consists of a single fistula, while in the 
former it is constructed of a basal mass from which numerous fistulae emanate. The 
fistular organs in each genus very closely resemble each other in their form and structure. 
Besides these structural differences, there are others, of a less striking description, that 
strongly indicate the necessity for generic separation. Thus in Alcyoncellum corhicula, 
in the Museum at Paris, and Euplectella aspergillum, Owen, there are an abundance of 
interstitial spicula of rectangulated sexradiate forms, which are very characteristic of 
those species, while the British species of Polymastia with which we are acquainted 
appear to be totally destitute of these complicated and beautiful forms of spicula. I have 
therefore thought it desirable, notwithstanding the close agreement that exists in the 
structure of their fistulee, that a generic distinction should be established between them. 
Halichondria mammillaris, Johnston, is the best type of the genus Polymastia. The 
whole of the parietes of these elongated fistulae are inhalant. In some specimens of 
P. mammillaris dredged in Vigo Bay by my friend Mr. M^^Andrew, the open pores are 
exceedingly numerous, and the exhalant organs are as distinctly shown to be confined to 
the distal extremities of the fistulae. 
