19 Desor on the Embryology of Nemertes. 
integral part of the vitellus, and being for a time the seat of 
a peculiar motion, should be considered as a peculiar evolu- 
tion of animal life, which though unnoticed until now, deserves 
none the less to be taken into serious consideration. 
Another point of no less importance is the existence of 
two kinds of ciliary motion, distinct from each other, and 
which, though simultaneous, are independent. While the 
vitelline sphere, as a whole, revolves, the embryo within is 
endowed with a peculiar motion performed by means of ciliæ, 
similar in their appearance to those of the envelop, and which 
continue the same motion after the animal is hatched. There- 
fore, there can be no doubt that this motion is a true volun- 
tary motion. The cilie of the envelop in this respect are 
essentially different, and their. motion may be considered as 
merely organic, similar in some respect to that of mucous 
membranes. 
Ciliary motion ought therefore no longer to be considered 
as a peculiarity of certain classes of animals, neither ought we 
to look at it as being exclusively connected with certain func- 
tions. It is a general motive agency that nature adapts to all 
sorts of functions, particularly to locomotion among the lower 
animals, and also among many higher animals in the embry- 
onic periods of their life. - 
ON THE DEVELOPMENT OF POLYNOE. 
Extraordinary as the development of Nemertes may ap- 
pear, it is not without some analogies in other Worms. We 
would especially refer to Polynée squamata, an Annelid very 
common on both shores of the Atlantic. To my friend 
M. Lovén are due the first investigations of the development 
of these worms.' His observations have lately been com- 
pleted by M. Sars, the distinguished Norwegian embryologist, 
! Several specimens were exhibited by Prof. Agassiz before the American Acade- 
my, in this state. 
