190 ANATOMY AND PHYSIOLOGY 



Hymedesmia, Bowerhank. 



Skeleton. A common basal membrane sustaining a thin 

 stratum of disjoined fasciculi of spicula. 



Type, Hymedesmia Zetlandica, Bowerbank. 



The species on which this genus is founded very closely 

 resembles in habit and general appearance those of the 

 genera Microciona and Hymeraphia, and in regard to the 

 special offices of the basal membrane, it assimilates with 

 them completely. But it differs from them, inasmuch as 

 the spicular portions of the skeleton do not emanate imme- 

 diately from the basal membrane, but are recumbent on it 

 in the form of disjoined fasciculi of spicula. But although 

 different from them in this important respect, the close 

 alliance with them is indicated by the common habit of the 

 possession by the basal membrane of the whole, or nearly 

 so, of the defensive spicula of the sponge ; indicating the 

 common property of extreme thinness of structure which 

 exists in these genera. 



The free condition of the fasciculi of the skeleton con- 

 nects this genus in some degree with the Halichondroid 

 genera of sponges, but there are none of the species of those 

 genera in which the fasciculi of the skeleton are separate 

 from each other. The nearest aUied genus in that direc- 

 tion appears to be Hymeniacidon. Eig. 371, Plate XXXV, 

 exhibits the disjoined fasciculi of the skeleton in situ, in 

 Hymedesmia Zetlandica, X 108 linear; and Kg. 296 

 Plate XVITI, represents a small portion of the inner surface 

 of the dermal membrane of the same sponge, showing the 

 fasciculation of the simple bihamate spicula, the equi- 

 anchorate ones dispersed singly on the membranes, and the 

 large attenuato-acuate entirely spined defensive ones in situ, 

 X 308 linear. 



