80 
Dr. Walltch, on the Polycystina. 
at right angles to the axis of the part, and never, as in the 
Spongid(B,\n two opposite directions around a central stolon. t 
In the embryonic portion of the skeleton of the Polycystina 
two distinct and very definite forms occur, which apparently 
never vary so far as to render their determination uncertain, 
either in the earliest or any subsequent stage of growth of the 
organism. To this embryonic skeleton I have given the name 
of the omphalostype,"^ and to the earliest-formed chamber, 
which is invariably formed around or upon the omphalostype, 
the name of the omphalic chamber. 
The two primary types of omphalostype may be conve- 
viently distinguished as the symmetrical and the asymme- 
trical — the one consisting of a hollow spherule of silex, which 
is minutely perforated just as the adult portions of the skele- 
ton ; the other_, of a series of continuous siliceous filaments, 
which I can only liken to the framework of a coronet. In 
other words, it may be said to consist of a basal ring, from 
one aspect of which loops are projected, which unite at a 
common centre considerably beyond the plane of the ring, 
and usually terminate at this apical point in a stout spine, 
whilst secondary loops and spines are projected from indefinite 
points, and these again give off filaments. As already stated, 
however, the primary rudiment of the skeleton, either when in 
the shape of the minute perforated spherule or the coronet-like 
framework, remains constant and determinable, no matter how 
exuberant or monstrous the growth may ultimately become. 
The importance of being readily able to discriminate 
between these two types depends on the fact of their corre- 
sponding to two well-defined and constant types of growth in 
the remainder of the siliceous skeleton, no matter what the 
genus or species may be : the symmetrical omphalostype 
invariably appertaining to those genera in which the growth 
of the parts takes place by the creation of fresh chambers 
arranged concentrically, or some portions of which are 
arranged concentrically, around the first or omphalic 
chamber : the asymmetrical, on the other hand, as invariably 
belonging to those genera in which growth takes place by the 
formation of new chambers from that aspect only which is 
opposite to the apex of the omphalostype. 
The characters of the embryonic skeleton of the Polycystina 
to which I refer are taken at the period when siliceous deposit 
first shows itself in the sarcoblast. This portion of the struc- 
ture consists of a minute spherical or nearly spherical mass 
of granular sarcode, yellowish in colour, and varying in 
diameter, as soon as free, from -rro-o'th to -i-jVoth of an 
* From ofi^aXoc, a navel, and ctvttoq, a stock or stem. 
