M. R. Leuckart on the Young States of some Annelides. 365 



appears now to be completely justified, although the determi- 

 nation of the species was incorrect. 



The connexion between the Mesotrocha of Trieste and the 

 Chcetopterus pergamentaceus is rendered in the highest degree 

 probable by the structure of the bristles. The same forms 

 which Busch has figured from the segments before the circlet 

 of cilia in his Mesotrocha^ have also been observed and described 

 by me on the anterior part of the body of our Chaetopterus, 

 Even the peculiar broad bristles which Busch found upon the 

 fifth segment of his larva reappear in the mature worm (but on 

 the fourth segment), only they are less distinctly serrated, and 

 occur in greater number, — as indeed the number of bristles in 

 general is much more considerable in the mature state. 



If we are once convinced of the identity of Mesotrocha and 

 Chatopterus, it becomes easy to refer the two forms to one 

 another, and thus, even without direct observation, to obtain 

 a tolerably complete view of the metamorphoses of the animal 

 in question. 



I have shown, as above quoted, that Chcetopterus is fur- 

 nished, like the other Annelida, with a true cephalic tubercle. 

 In the developed state this part is certainly very rudimentary, 

 so that it might previously have been overlooked ; but in the 

 larva it is very greatly developed, and appears produced over 

 the mouth into a disc, like an upper lip*. To this upper lip in 

 the larva a bilobed lower lip is opposed, the two lobes of which 

 only require to be fused together to form the peculiar labial 

 apparatus of the mature animal. The two tentacles placed in 

 our larva at the sides of the mouth also occur in Chcetopterus^ 

 according to my observations, although much less developed in 

 proportion, and, to a certain extent, aborted, like the cephalic 

 tubercle and the eyes. 



The segments which in the larva intervene between the head 

 and the circlet of cilia form (together with the head) the fore- 

 body of the mature animal. But their number still requires 

 increase; Busch only counted nine segments, whilst in my 

 worms there were twelve or thirteen. The new segments are 

 evidently formed at the hinder extremity of the fore-body; 

 that is to say, between the fore- and middle body, as suffi- 

 ciently appears from the position of the above-mentioned ser- 

 rated bristles ; this, as far as I know, is the only instance of the 

 kind amongst the Annelida. With the last segments of the 

 fore-body, Mesotrocha is also destitute of the large wing-like 



* From the formation of the head in the larva, it is established beyond 

 a doubt that Sars has really, as I formerly stated, taken the back of Cha- 

 topterus for its belly. 



Ann. ^ Mag. N. Hist. Ser. 2. Vol. xvi. 18 



