M. R. Leuckart on the Young States of some Annelides. 271 



but to some other, unknown, but very nearly allied worm. Since 

 I have become better acquainted with the metamorphosis of the 

 Mesotrocha sexoculata from Miiller's statements, I must now 

 agree with him entirely in this opinion. The course of deve- 

 lopment of Mesotrocha, which I took as the basis of my argu- 

 ments, and could only deduce from the comparison of Meso- 

 trocha with the mature Chtetopterus, certainly agrees in general 

 with this process as observed by Miiller ; but I was wrong in 

 supposing that the lenticular processes of the middle segments 

 of the body were produced by a fusion of the lateral dorsal ap- 

 pendages which occur in the larva described by Busch. The 

 remarkable form of the segments in question is the result rather 

 of a peculiar development of the bodies of the segments, without 

 the participation of any segmental appendages, as indeed I had 

 previously assumed, in opposition to the assertion of Audouin, 

 in my description of Chcetopterus j^ergamentaceus. On the other 

 hand, M. Miiller was able to convince himself, that the large 

 wing-like pi-ocesses on the last segments of the anterior part of 

 the body in Ch^stoptei-us appeared at first only as simple dorsal 

 processes, and it is remarkable that, like the dorsal processes of 

 Busch's larva, they are immediately behind the circle of cilia, 

 which, as is well kno\\'n, is double in Mesotrocha sexoculata. It 

 is possible therefore, that in the worms like Chetopterus, for a 

 knowledge of the larval state of which we are indebted to Busch, 

 four or more such appendages occur on each side instead of a 

 single one ; and it is also possible certainly, that the dorsal pro- 

 cesses of the larva may really, as supposed by Busch, become 

 gradually developed into branchiae. 



From the statements of M. Miiller, it also appears that the 

 characteristic distinctions between the two species of Chatopterus 

 at present known are by no means of such a decided nature as 

 one might perhaps have concluded from the difference in their 

 size, and as was presupposed by me in the preceding analysis. 

 This applies especially even to the structure of the bristles, which 

 appear to be essentially similar in the two species, so that in my 

 paper the name of C. norvegicus might be substituted throughout 

 for C. pergamentaceiis with equal right. 



Under these circumstances I can of course only bespeak some 

 consideration for my statements, in as far as they may place be- 

 yond a doubt the relations of the larva described by Busch to a 

 {still unknown) form of the family of the Chaetopteridse. 



