PHYSIOLOGY OF THE SPONGIADAE. 
1101 
the tissues, and differing in no respect from the surfaces of any of the smaller internal 
cavities of the sponge. I am therefore inclined to consider such excavations as abnor- 
mal occurrences, which are not entitled to be considered as of either generic or specific 
value. Part II. Plate XXXII. figs. 2, 3, & 4; and Plate LXXII. fig. 5. 
Pachtmatisma, Bowerbank. 
Skeleton composed near the external surface occasionally of short fasciculi of siliceous 
spicula, disposed in lines at about right angles to the surface of the sponge. Central 
portion of the sponge unsymmetrical. Dermis crustular, furnished abundantly with 
closely packed ovaria. Ovaria siliceous, formed of cuneiform spicula, firmly cemented 
together in lines radiating from the centre of the ovary. Pores furnished with 
oesophageal tubes, terminating in the distal extremity of each intermarginal cavity. 
Intermarginal cavities symmetrical, subcylindrical, with a pyloric valve at the 
proximal end of each. 
Since the first publication of my description of the sponge on which this genus is founded 
in the Synopsis Spongiarum of Dr. Johnston’s ‘ History of British Sponges,’ p. 243, I 
have found it necessary to base the generic characters of the Spongiadse on the structural 
peculiarities of the skeleton and reproductive organs. I have therefore reconstructed 
the character of the genus in accordance with this rule. 
This genus is closely allied to Geodia in its organic structure, but the difference in 
the arrangement of the skeleton readily distinguishes them. The general aspect of the 
species of each genus is also strikingly distinct. I am acquainted with six species of 
Geodia and three of PachymaUsma ; and in every case the species may be readily referred 
to its proper genus even by its general aspect. All the species of either genus have a 
crustular dermis, and the structures of the ovaria are also alike in each. I have described 
the anatomical peculiarities of the latter organs so fully in the description of the generic 
characters of Geodia as to render it unnecessary to treat of them here. Plate LXXII. 
fig. 6. 
Ecionemia, Bowerbank. 
Sponge having a strong axial column or centre of closely packed siliceous spicula 
disposed in lines parallel to the long axis of the sponge, from which axial column 
or centre a peripheral system of spicula radiates at about right angles. Distal ends 
of the radii furnished more or less with ternate connecting spicula, the radii of 
which are disposed immediately beneath the dermal membrane. 
This genus differs from Piety ocylindrus in having the axial column composed of a 
dense mass of parallel spicula instead of a column formed of an open network of spicula ; 
and the peripheral system is also different, inasmuch as it is essentially a portion of the 
interstitial system of the sponge, and not more especially a defensive system as it 
appears in Pictyocylindrus, in no species of which genus have there ever yet been found 
temate spicula at the surface, while in Pcionemia acervus., the type species of the genus, 
they are abundant. 
