OF THE SPONGIAD^E. 189 



Genus HYMERAPHIA, Bowerbank. 



Skeleton. A single basal membrane, whence spring nu- 

 merous large separate spicula, which pass through the 

 entire thickness of the sarcodous stratum to, or beyond 

 the dermal surface of the sponge. 



Type, Hymeraphia stellifera, Bowerbank. 



This genus is nearly allied to Microciona, but is more 

 simple in its structure ; as in place of the columns of the 

 skeleton compounded of keratode and spicula cemented 

 together, and emanating from a common basal membrane 

 as in the latter genus, we find single spicula only, devoid 

 of keratode and based on a common membrane, whence 

 they pass through the entire substance of the sponge ; and 

 in all the species at present known, they penetrate the 

 dermal membrane and project beyond its surface to a con- 

 siderable extent, thus combining the two offices of skeleton 

 and external defensive spicula. These organs are there- 

 fore, as compared with the skeleton spicula of other mem- 

 bers of the Spongiadse, and to the entire mass of the 

 sponges to which they belong, of exceedingly robust pro- 

 portions ; their length being frequently twice that of the 

 entire thickness of the sponge. 



These peculiarities of structure indicate a common habit 

 of extreme thinness in the species, and such is in reality 

 the condition of those with which we are acquainted. 

 Fig. 370, Plate XXXIV, represents a section of Hymera- 

 plua stclUfcra, Bowerbank, showing the large bulbous 

 skeleton spicula in situ, their apices forming the external 

 defences ; a, the stelliferous internal defensive spicula ele- 

 vated by a grain of sand beneath the basal membrane, 

 X 108 linear. Fig. 34, Plate I, exhibits one of the stel- 

 liferous defensive spicula, X 260 linear. 



