1110 
DE. J. S. BOWEEBANK ON THE ANATOMY 
the sides of which spring stout, long, curved, fusiformi-attenuato-subspinulate spicula, 
the convex side of each spiculum being outward ; and each column terminates with five 
or six of these spicula disposed in the same manner and at the same angle to the axial 
line of the column, that is, from about twenty to forty-five degrees. The proportions of 
the skeleton-columns vary in different species. In M. atrosanguinea they are short, 
stout, and exceedingly well defined. In M. amhigua they are short and indistinctly 
produced, and in M. carnosa they are long, slender, flexuous, and frequently branched ; 
but however they may vary in their proportions in different species, their normal 
character, both as regards structure and position in the sponge, is always preserved. 
Plate LXXIV. fig. 2, and Part II. Plate XXX. figs. 1 & 2. 
Genus Hymeeaphia, Bowerbank. 
Skeleton a single basal membrane, whence spring numerous large separate spicula, 
which pass through the entire thickness of the sarcodous stratum, to or beyond 
the dermal surface of the sponge. 
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 considerable 
extent, thus combining the two offices of skeleton and external defensive spicula; 
These organs are therefore, as compared with the skeleton-spicula of other members of 
the Spongiadse, and to the entire mass of the sponges to which they belong, of exceed- 
ingly robust proportions, their length being frequently twice that of the entire thick- 
ness 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. 
Part II. Plate XXX. fig. 3. 
Hymedesmia, Bowerbank. 
Skeleton a common basal membrane sustaining a thin stratum of disjoined fasciculi of 
spicula. 
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 immedi- 
ately from the basal membrane, but are recumbent on it in the form of disjoined fasci- 
culi 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. 
