M. Sars on the Development of the Annelides, 183 



XX. — On the Development of the Annelides. By M. Sars*. 



[With a Plate.] 



Till 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 Polyno'e cirrata, Fab., that the young when they leave 

 the Q^^ 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, 1 could not succeed in 

 tracing the development any further; and it might, perhaps, appear 

 superfluous to publish these observations at present, after Loven 

 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 in which our 

 knowledge of the development of the Annelides is still deficient. 



Polyno'e cirrata is common on the coast of Norway, and occurs 

 between the roots of Laminarice, 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. 



