186 M. Sars on the Development of the Annelides. 
to expand considerably from the very mouth and to form a large 
sac, the stomach, and then gradually narrowing to proceed to- 
wards the hinder part of the body, where probably the anal aper- 
ture is situated. I could not distinctly recognise this, but I have 
observed it very clearly at this place m similar young of another 
Annelide, which will be noticed subsequently (fig. 21f). The 
colour is everywhere of a dirty pale green and only slightly trans- 
parent. The body is soft, but it rarely exhibits contractions or 
variations of form ; it is only when the young animal is quiet, or 
has but little water, that contractions are perceptible on its body 
(and sometimes also of the intestinal canal), from its becoming 
broader or narrower and curving slightly at some places. 
Locomotion, that is to say swimming, is effected by the vibra- 
tion of the cilia. Only the large cilia of the circle effect the loco- 
motion ; the small ones near the mouth and at the front extremity 
of the head contribute little or nothing to it. The former cor- 
respond therefore to the powerful cilia which, in the young of the 
Nudibranchize and many other Gasteropods, effect the swimming, 
and are subject to the will of the animal ; the latter, on the other 
hand, are not subject to their will, and constitute the so-called 
ciliary organs. 
During the swimming, which is very rapid, uniform, and in 
all directions, the anterior portion of the head (fig. 18 0) is always 
directed forwards. Frequently these young animals revolve du- 
ring swimming round their longitudinal axis: their sight is di- 
stinctly developed, for they are seen to avoid one another with 
adroitness, and they always swim towards the light. Although 
I turned the glass contaiming an immense number of them in 
various ways, they immediately swam in great troops to the side 
turned towards the light. 
The time from the laying of the eggs till the extrusion of the 
young may probably amount to a couple of weeks, for I have 
found, in the first days of February, the cavities of the body of 
a Polynoé filled with eggs; but from the middle of this month 
to the middle of Mar ch, exes on the backs in some individuals ; 
and in others at this same “period, young just on the point of 
quitting the backs of their mother (fig. 12 aa). 
I kept the above-described young of Polynoé, which left the 
egg under my eyes, alive for four weeks in glasses filled with sea- 
water, during which time they grew it is true somewhat, but ex- 
hibited no further changes. In this respect Loven was more 
fortunate, for the young Annelides which he met with swimming 
freely in the sea were evidently further advanced, and therefore 
exhibited to him in the space of two days the further development, 
the tentacula, and the articulations of the body growing under 
