266 M. II. Leuckart on the Young States of some Annelides. 



appendages, which, according to my view, are produced by the 

 metamorphosis of the last pedal tubercles of that part of the 

 body, as also of the uncini which I have indicated on its last 

 two segments. 



The second division of the body in Mesotrocha, which com- 

 mences with the circle of cilia, represents the middle and hinder 

 parts of the body of our Chcetopterus, but apparently requires a 

 still greater increase for its complete development, as Busch only 

 counted eleven segments in this part of his larva, whilst in the 

 mature state more than double the number occurs. When, 

 however, Busch denies the possession of setose feet to this divi- 

 sion of the body, he probably only had the subulate bristles of 

 the anterior part of the body in his eye, for the eleven pairs of 

 rudimentary feet with " longish oval bodies " which are inserted 

 in them, undoubtedly represent setose feet, and are probably 

 the first indications of the ventral tubercles furnished with 

 uncini. Busch even states, that the inserted bodies are den- 

 ticulated, as I have found to be the case with the imcini of 

 Ch(jEto])terus. It is interesting also that these ventral pedal 

 tubercles possess a bilaterally symmetrical distribution on all 

 the segments, whilst in the anterior segments of the middle part 

 of the body they are subsequently, as is well known, fused into 

 a single median organ. The dorsal tubercles which occur on 

 the abdomen of the mature Chatopterus appear to be developed 

 subsequently. 



My regarding the segments lying behind the circles of cilia 

 as representing both the middle of the body and the abdomen 

 of Chatopierus, arises from the obsei*vation of Busch, that the 

 dorsal surface of these segments is gradually produced on each 

 side, close to the median line, into a tolerably long and vibrating 

 process. Busch considers these processes to be branchiae, and 

 concludes accordingly that Mesotrocha is the larva of a Dorsi- 

 branchiate Annelide ; he consequently assumes that these pro- 

 cesses are persistent in their original form, whilst in my opinion 

 (which however is only founded on the supposed identity of 

 our larva with the Chatopterus of Trieste) they are gradually 

 converted, by growing together in the middle, into the scale-like 

 appendages which communicate to the segments of the middle 

 of the body, and also, although much less remarkably, to the 

 first segments of the abdomen, their well-known lenticular 

 or conical form. The development of these processes com- 

 mences close behind the rings of cilia, and gradually extends 

 thence backwards, — how far has not yet been observed. The 

 oldest larvse of Mesotrocha which presented themselves for exa- 

 mination, exhibited these processes fully developed only on the 

 first four segments ; consequently, exactly on those segments 



