OP THE SPONGIADiE. 177 



extremity of the sponge; the essential difference being 

 that in Grantia the distal end of the cloaca is open, and in 

 AhyoTicellum 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 justified 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 {Euplectella 

 aspergillum, Owen) especially is furnished 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 cha- 

 racters it agrees exceedingly closely with A. aspergillum. 



Pig. 356, Plate XXIX, is a view of a small portion of 

 the surface of Mr. Cuming's specimen of Alcyoncellum 

 aspergillum, exhibiting the mode of disposition of the in. 

 halant areas ; a,ikQ primary fasciculi of the skeleton ; b, 

 the secondary fasciculi, plus about 5 Hnear. Pig. 357 re- 

 presents the congregated oscula within their marginated 

 area at the distal termination of the sponge. Natural 

 size. 



Polymastia, Bowerhank. 



Skeleton. Basal mass. Central portion consisting of a 

 plexus of contorted anastomosing fasciculi, resolving 

 themselves near the surface into short straight bundles 



12 



