OF THE SPONGIADjE. 187 



composed of numerous spicula ramifying continuously, and 

 each ramulus increases in size and the number of its 

 spicula as it approaches the surface of the sponge. Single 

 spicula are frequently projected from the ramuli in an 

 ascending direction at an angle of a few degrees, and at 

 their distal terminations at the surface of the sponge ; 

 the whole of the terminal spicula radiate more or less at 

 angles from, their axial line, and passing through the 

 dermal membrane form the external defences of the 

 sponge. Although constantly ramifying and freely inter- 

 mingling, I have never detected them anastomosing. The 

 term Pkakettia is applicable to both the primary and 

 secondary ramifications of the skeleton. The type of this 

 genus is Halicbondria ventilabrum, Johnston. I have not 

 yet met with an exotic species of the genus. Fig. 367, 

 Plate XXXIII, represents a longitudinal section of one of 

 the primary radial lines of the skeleton structure, exhibiting 

 the slender secondary radiations of the skeleton ; a, part of 

 the primary axial portion of the skeleton ; b, dermal mem- 

 brane, X 50 linear. 



The genera Microciona, Hymeraphia, and Hymedesmia 

 form a group essentially different in structural character 

 from the other genera of the Spongiadae ; but they are 

 closely allied to each other by the peculiar characters of 

 their basal membranes in conjunction with the other parts 

 of the skeleton. From the nature of their structures, the 

 species generally assume a thin coating form and are often 

 very minute. 



In most of the genera of SpongiadsB the basal membrane 

 of the sponge ceases to be of marked importance after the 

 earliest stages of its development, but in these genera it 

 continues throughout the whole existence of the sponge to 

 form an important part of its skeleton structure. It is a 

 common base whence spring the whole of the other com- 

 ponent parts of the skeleton ; and its importance is further 

 indicated by its also being the common base in some 

 species of the internal as well as the external defensive 

 picula of the sponges in which those organs occur. 



