M. Sars on the Development of the Annelides. 183 
XX.—On the Development of the Annelides. By M. Sars*. 
[ With a Plate. ] 
Tit recently, all that was known respecting the development 
of the Annelides was based solely upon observations made on the 
leech ; the other Annelides were judged of from this, and their 
development considered to be extremely simple, i. e. that the 
animals left the egg as perfectly formed as they appear during 
their whole life. To what very erroneous conclusions we fre- 
quently come in this way, and how cautious we ought to be in 
generalizing, abundant instances prove. So, for instance, not to 
mention others, it was concluded, from the knowledge of the de- 
velopment of the craw-fish, that all the other Decapods were in 
this respect similar ; and naturalists were thus led to doubt for a 
long time, to the injury of science, the beautiful discoveries of 
Thompson. 
In the month of February 1840 I discovered, in the examina- 
tion of a Polynoé cirrata, Fab., that the young when they leave 
the egg have a very different form from that of the adult animal, 
and that they are deficient in most of the external organs which 
are so characteristic of these animals ; in a word, therefore, that 
this Annelide is subject to a metamorphosis. I only succeeded in 
observing the first stage of development, and therefore kept back 
my observations on this subject, with numerous other imperfect 
notices, with the hope of being able to complete them in the 
course of time. However, although I had occasion to repeat the 
observation in February and March 1841, I could not succeed in 
tracing the development any further; and it might, perhaps, appear 
superfluous to publish these observations at present, after Lovén 
has communicated to the public his far more complete observa- 
tions on the metamorphoses in a species of Annelides. I do it 
however partly to confirm the latter, which no one yet has done, 
and partly because I am able, which was not the case with Loven, 
to point out a known species in which at a certain period of the 
year the development may be observed. When the minute cir- 
cumstances or conditions in the generation are once known, some 
one will undoubtedly succeed in completing that m which our 
knowledge of the development of the Annelides is still deficient. 
Polynoé cirrata is common on the coast of Norway, and occurs 
between the roots of Laminaria, under stones, in empty shells and 
other holes in which it can hide itself. It agrees perfectly, as I 
have convinced myself by comparison, with the Greenland species 
characterized by Fabricius under this name, but it never attains 
on our coast the immense size it does on that of Greenland. 
In the months of February and March is the period of propa- 
* From Wiegmann’s Archiv, 1845, Part I. 
