Am) PHYSIOLO&T OF THE SPONGIADiE. 
813' 
condition. Fig. 11, Plate XXXIII. represents two of the cases after treatment with 
acid, one of them (d) having the ovary very much reduced in size by the dissolution of 
the thick coriaceous portion of its structure. 
In the second group of the ovaries of the Spongilladse there is also a strong general 
resemblance in structure to the type-form of /S. lacustris, but each species is distinctly 
characterized by peculiarities of form and arrangement of the spicula. 
The normal form is spherical, and the walls of the ovaries, in six out of the seven 
species with which I am acquainted, are comparatively thin. In the , seventh species, 
/S. Carteri, Boweebaxk [S. friabilis, Caetee), they are very thick and abundantly fur- 
nished with cellular structure, arranged in lines radiating from the centre to the circum- 
ference ; each line consists of nine or ten cells, the length of each being about equal to 
the diameter. They are very closely packed together, and are irregularly angular by 
compression. Their combined length varies from about one-fifth to one-sixth the length 
of the diameter of the ovarium. This is the only species in which I have detected this 
description of cellular structure. Fig. 16, Plate XXVIII. represents a portion of the 
surface and a view of the cells in situ. 
Although the spiculated coriaceous form of ovarium prevails so constantly among the 
freshwater sponges, it is one of extremely rare occurrence among the marine species ; 
and I have met with only one instance of its occurrence, and that is in a new genus of 
sponges from Shetland, for W'hich I am indebted to my indefatigable friend Mr. Baelee. 
The specimen incrusts a portion of the valve of a Pecten, covering a space about half 
an inch in length and the eighth of an inch in breadth, and it does not exceed half a 
line in thickness. The ovaries are numerous and closely packed together, and are 
distinctly visible to the unassisted eye, looking like very minute cocoons of some terres- 
trial insect. There were nearly thirty in an area equal to about a quarter of an inch. 
They are attached by the sides to one or more branches of the fibrous portion of the 
skeleton. 
The wall of the ovary is very thin, and’ appears to consist of a single membrane pro- 
fusely furnished with acerate spicula, like those of the skeleton. They cross each other 
in every possible direction, and occasionally appear to assume a somewhat fasciculated 
arrangement. The ovaries are not uniform in shape, some being regularly oval, while 
others are more or less ovoid. I could not detect any trace of a foramen in those I sub- 
jected to examination. I have designated this interesting species Piplodemia vesicula 
in my MS. description of it. Fig. 1, Plate XXXIV. represents two of the ovaries in 
their natural condition after immersion in Canada balsam, magnified 83 linear. 
In the genera Geodia and Pachymatisma ovaria are produced in great abundance. 
They agree in form very closely with those of Spongilla, but their structure is widely 
different, and the soft animal matter that enters so largely into the structure of those 
of the freshwater sponges scarcely makes its appearance in the ovaries of Geodia, their 
walls being composed of closely packed spicula, firmly cemented together by silex. 
Their situation in the animal is also different from those of S^ongilla, in which they are 
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