M. Sars on the Development of the Annelides. 185 
form of a membrane, similar to the so-called membrana nidulans 
of Burdach, connects all the ova as it were by means of a tena- 
cious mucus. ‘This fringe is seen now and then to move slowly, 
and curve in a worm-lke form, drawmg the egg with it back- 
wards and forwards. The cause of this motion remains a 
mystery to me, if it be not owing to the action of the water on 
the mucous substance of the fringe. The foetus itself, which gra- 
dually acquires a bright grayish green colour, was still without 
motion in most of the ova; only in a few a circle of extremely 
minute, projecting and vibrating cilia was perceptible, which sur- 
rounds horizontally the centre of the body of the foetus at an 
equal distance from the two poles of the ovum. 
At last the foetus is arrived at maturity, and the mother now 
carries on its back many thousands of young ones (fig. 12 aaa), 
which gradually come forth from the mucus surrounding the eggs, 
leave their mother and swim freely about in the water, visible to 
the naked eye us very minute greenish gray points (z\,th of a 
millimeter m size) endowed with a lively motion. 
The young, which have just left the shell (fig. 18, 19), are ex- 
tremely unlike the mother both in form and m structure. They 
are short, oval, cylindrical, unarticulated, and have, as above men- 
tioned, horizontally round the centre of the body, a-circle of tole- 
rably long cilia (dd), in other respects however without any ex- 
ternal organs. The portion of the body situated anteriorly to the 
ciliary circle is somewhat narrower than the hinder one, and 
bears two eyes (ee), and should therefore without doubt be con- 
sidered as the head, the more so as the young one always swims 
with this extremity in front. The eyes are at some distance from 
the anterior free extremity (b), in the vicinity of the circle of cilia, 
one on each side and a little towards the back; they are very 
large in proportion to the body, black, and slightly elongated 
diagonally, or almost kidney-shaped, with the convexity turned 
anteriorly ; not a trace of tentacula or antenne is observable 
about the head. 
We just now called the side where the eyes approach nearest 
together the dorsal side, while the opposite one, which moreover, 
when the young is regarded from the anterior extremity, is some- 
what more projecting (fig. 19 @),1s proved to be the ventral side, 
from the fact, that on it, close behind the circle of cilia, there is 
an aperture (fig. 18 @), which I look upon to be the mouth. This 
mouth-aperture is a diagonal fissure, whose lips are provided with 
vibratory cilia, which are however much smaller than those of the 
circle of cilia. There are also some very minute cilia at the most 
anterior extremity of the head (fig.18 4). The intestine, as far as I 
could observe from the slight transparency of the body, appears 
