ON FRESHWATER POLYZOA. 325 
incompressible nature of these ova, such an orifice, were it present, could hardly 
escape detection. Meyen%, it is true, states that he has witnessed in Alcyo- 
nella fungosa the escape of an egg through an opening in the vicinity of 
the anus; but, notwithstanding a similar observation already noticed as made 
by Van Beneden on the marine Laguncula repens, this I feel certain has been 
an imperfect observation of Meyen, and that the escape of the egg was the 
result of some accidental laceration of the tissues in this spot. There is then 
no natural aperture through which the ova can escape, and their liberation 
_ Iam convinced can only take place after the destruction of the soft parts of 
a Polyzoon has afforded to them a mode of egress through the mouth of 
the cell. 
3. Reproduction by free Embryos.—While engaged in the examination of 
a specimen of Plumatella fruticosa, I observed in the water which contained 
it, a small egg-shaped body of a white colour. On placing this under the 
microscope, I distinctly saw through the transparent external skin that it 
enclosed a young polyzoon, and on now rupturing this skin with the point of 
a needle, the little polyzoon was set at liberty. ‘This consisted of a solitary 
well-developed polypide enclosed in a completely formed cell, frem which it 
every now and then protruded the upper part of its body by a process of 
evagination, just as in the adult animal. The cell appeared to consist of the 
endocyst alone, and the whole of its external surface to the line of invagina- 
tion was densely clothed with long vibratile cilia. The little animal would 
sometimes remain stationary with the upper part of the polypide protruded 
from the cell, but most constantly the entire polypide was retracted and the 
mouth of the cell closed, and then the little embryo would present the ap- 
pearance of a minute sphere covered with long cilia, by whose action it was” 
carried about through the water with rapid and elegant motion. 
Some time after discovering the free embryo of Plumatella fruticosa, I 
observed similar locomotive bodies in the water in which I had been keeping 
a specimen of Alcyonella fungosa. These, in the retracted state, were of a 
more elongated form than the little embryos already described, from which 
they moreover differed in invariably containing two polypides in a single 
cell. They were exceedingly active in their motions, moving always with 
the eul-de-sac of the cell foremost, and at the same time revolving on 
their axis in an exceedingly elegant manner; they frequently assumed a pear- 
shaped figure, with the narrow end corresponding to the orifice of the cell. 
__ In both the embryos now described the general structure is quite similar. 
A soft, transparent and eminently contractile sac, partly clothed with cilia, 
has the unciliated portion invaginated into the ciliated, down which it extends 
for some distance, and then turning back upon itself is reflected upwards, 
when it experiences another invagination before becoming attached beneath 
the tentacular crown of the polypide. The first invagination is rendered 
_ permanent in this stage of the embryo by numerous bands, so closely re- 
sembling the superior and inferior parieto-vaginal muscles of the adult as to 
lead at first to the belief that they are these very muscles visible in the em- 
bryo. Such however is not the case. Neither these bands, nor the invagi- 
nation with which they are connected, have any existence in the adult; and 
it is the second invagination just mentioned which the latter alone retains. 
The great retractor muscle of the polypide is well developed in the embryo. 
The subsequent development of the embryo I have not been able exactly 
to follow ; it seems however probable that it consists in the obliteration of 
the inferior invagination, and the disappearance of the cilia from the surface 
of the sac, with the formation of an ectocyst, the embryo at the same time 
* Loc. cit. 
