32 Mr. H. J. Carter on the Ovigerous Layer of 
situated on the most pendent or prominent part, which is the 
outlet of a well-developed excretory canal-system, whose 
branches pervade the extremely fine and compact structure of 
which the sponge is otherwise composed. The pinlike 
spicules &c. and the ovigerous layer that I am about to 
describe are also particularly evident; but as the sponge 
generally has been heretofore repeatedly noticed with the 
exception of this layer, I shall henceforth confine myself to a 
description of the latter only. 
If a vertical section of one of these specimens be made, so 
that the incision may fall perpendicularly and longitudinally 
on the shell over which the sponge may have grown, and the 
two portions forcibly separated trom each other, so as to ex- 
pose the shell beneath (say a Turritella about 14 inch long), 
a yellowish chitinous layer composed of ova closely packed 
together (not unlike the nidamental layer of a mollusk) will 
be left upon the shell, corresponding in extent to the amount 
of the shell covered by the sponge, whether this be a part or 
the whole, and adhering so strongly that the whole of the 
sponge-substance may be washed off with a brush without dis- 
turbing the attachment (fig. 3). On the other hand, sup- 
posing that the sponge, as is commonly the case, be attached 
to the whole length of the TZurritella and the specimen 
(having been preserved in a wet state) is put into dilute nitric 
acid, the shell part will be entirely dissolved away, leaving 
the ovigerous layer in this instance attached to the sponge, 
when a similar section with a sharp thin knife may be made 
to pass through both the sponge and the ovigerous layer 
together ; and thus, by examining the object in water under a 
microscope, the thickness and structure of the latter may be 
easily ascertained. 
The ovigerous layer may then be observed to be composed 
of a yellowish tough chitinous stratum of ova in juxtapo- 
sition, but only one ovum deep or thick (fig. 2), of which 
the part that was in contact with the shell is flat, thin, 
and even (fig. 2,4), but that towards the sponge thick and 
granulated by the convexities of the layer of ovarian cap- 
sules, which, from being compressed together, vary slightly 
in height, size, and shape, so that, although generally the 
stratum or ovigerous layer is only one 200th of an inch thick, 
the thickness towards the sponge is so far rendered irregular 
(fig. 2, a), while the horizontal diameter of the ova when 
viewed in a flat position on the sponge side is found to vary 
from the 180th to the 90th of an inch (fig. 8); hence the 
ovum, being flat below, convex above, and rendered more or 
less polygonal laterally by horizontal compression, fails to 
