BRITISH SPONGIADiE. 291 



sponge is remarkably variable in its form ; one of the 

 specimens from Orkney is in the shape of a short irregular 

 cone, one inch and a half in height, with a large fistulous 

 cavity extending from the centre of the base to the apex, 

 with numerous small oscula on the sides of the cavity, 

 which is about three lines in diameter. The second speci- 

 men is a compressed mass, rather less than an inch in 

 height, one and a quarter inch in length, and half an inch 

 in thickness, having four large oscula on the distal ridge 

 nearly equidistant, and a fifth osculum terminating a short 

 curved columnar branch, thrown off one side of the sponge 

 midway between its base and apex, and partially impressed 

 on its side. The third specimen from Exmonth is para- 

 sitical, running irregularly over a small portion of the 

 surface of a specimen of Halichondria panicea, about one 

 and a half inch in length. It follows the sinuosities of the 

 surface of the sponge, to which it adheres without be- 

 coming elevated from it at any part, although about half 

 an inch of its substance has a compressed columnar form, 

 like that projected from the side of the second sponge 

 described above. One small osculum only was apparent, 

 and this specimen is evidently not fully developed. 



In all the specimens the pores were barely visible to the 

 unassisted eye. Many of them were, comparatively speak- 

 ing, of larger size. They are not indiscriminately dis- 

 persed, but are collected in numerous groups of irregular 

 size and form, and their areas are usuallv destitute of 

 dermal spicula. These peculiarities of the dermal tissues 

 form excellent distinctive characters for separating this 

 species from Isodictya densa, in the dermal membrane of 

 which species no such membranous areas exist. In other 

 respects the two species resemble each other closely in 

 their structural characters ; the spicula of each are of the 

 same length, but those of /. densa are much the stouter of 

 the two. The dermal membrane is pellucid, and I could 

 not detect any spicula imbedded in its surface excepting 

 those of the skeleton immediately beneath it : but the 

 interstitial membranes of the interior are scantdv furnished 

 with slender acerate tension spicula. 



