PHYSIOLOGY OP THE SPONGIAD^. 
1109 
1 know of no other species, either British or foreign, that possesses the peculiar con- 
formation that distinguishes the sponge that is the type of this genus. The primary 
cylindrical axes very closely resemble those of Bictyocylindrus ; but in that genus the 
spicula radiating from the axes are separate and distinct, each having its proximal end 
based on the primary cylinders of the skeleton, and its distal one reaching nearly to, 
or passing through, the dermal membrane of the sponge ; or if they be fasciculated, the 
fasciculi are simply plumose, and in no case with which I am acquainted at all ramu- 
lose. In Phakellia the secondary skeleton is formed of distinct slender branches, each 
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 intermingling, I have never detected them 
anastomosing. The term Phakellia is applicable to both the primary and secondary rami- 
fications of the skeleton. The type of this genus Halichondria ventilahrum, Johnston. 
I have not yet met with an exotic species of the genus. Plate LXXIV. fig. 1. 
The genera Microciona, Hymeraplna, and Hymedesmia form a group essentially 
different in structural character from the other genera of the Spongiadee ; but they are 
closely allied to each other by the peculiar characters of their basal membranes, in con- 
junction 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 Spongiadae 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 
in some species the common base of the internal as well as the external defensive 
spicula of the sponges in which those organs occur. 
Microciona, Bowerbank. 
Skeleton a common basal membrane, whence spring at or about right angles to its 
plane numerous separate columns of spicula intermixed with keratode, furnished 
externally with spicula which radiate from the columns at various angles towards 
the dermal surface of the sponge. 
The skeleton of the type of this genus, M. atrosanguinea, is different from that of any 
other genus of sponges that I have hitherto seen. It consists of numerous nearly 
equidistant, short, straight separate columns of spicula and keratode, from all parts of 
