86 Mr. H. J. Carter on the Freshwater Sponges of Bombay. 
burst or tear them from their attachments and force them to the 
surface of the water. Since then the coarse structure of plumosa 
would appear to offer greater resistance to the escape of this air 
than that of any of the other species, it is probable that this 1s 
the reason of my having hitherto only found it in the position 
mentioned. As Sp. alba, without its specific differences, is but 
a coarse form of cinerea, so plumosa is, without its specific differ- 
ences, only a coarse form of Sp. Meyeni. The point which di- 
stinguishes it from all the other species consists in the form of 
its seed-like bodies, which are ovoid. From Sp. Meyeni it is also 
distinguished by its surface being more even, its projections less 
prominent, and its tendency to spread horizontally more than to 
rise vertically. 
General Observations.—It should be stated that in all these 
species except cinerea, their forms en masse are so diversified and 
so dependent on accidental circumstances, that not one of them 
can be said to possess any particular form of its own, or to be 
distinguishable from the rest by it alone. 
The measurements of the seed-like bodies and spicula are taken 
from the average of the largest of their kind; they differ a little 
from those mentioned in my “ Notes*,” but this is owing to their 
having been the means of a larger number of measurements than 
I had an opportunity of making im the first mstance. How- 
ever great the number of measurements, it is probable that when 
made at different times and from different sets of specimens, the 
results will always somewhat differ ; but this is a matter of very 
little consequence, as these points alone are not required for 
distinguishing characters. 
The large spiculum is of the same shape in all the species, and 
is therefore of no use as a specific character. (Plate V. fig. 2.) 
Structure and Development. 
The freshwater sponge is composed of a fleshy mass, supported 
on a fibrous, reticulated horny skeleton. The fleshy mass con- 
tains a great number of seed-like bodies in all stages of develop- 
ment, and the horny skeleton is permeated throughout with sili- 
ceous spicula. 
When the fleshy mass is examined by the aid of a microscope, 
it is found to be composed of a number of cells imbedded in and 
held together by an intercellular substance. 
These cells vary in diameter below the +,/55th part of an inch, 
which is about the average linear measurement of the largest. 
If one of them be selected for observation, it will be found to be 
composed of its proper cell-wall, a number of granules fixed to 
its upper and inner surface, and towards its centre generally one 
or more hyaline vesicles. 
* Op. cit. 
