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



this also that Quatrefages has derived the name of his species. 

 The length of the developed animal is from 2|^ to 3 inches, — that 

 of the two tentacular cirri, 9 lines. The only thing that might 

 perhaps be urged with reason against the identity of the two 

 animals, is the circumstance that the first segment of the body, 

 according to Quatrefages, is destitute of the dorsal pedal tubercle ; 

 but w ith regard to this point of difference I should be rather in- 

 clined to suppose that there is an error of some kind. Eyes are 

 wanting in the developed animal, as in many other mature 

 w^orms (and Mollusca) which possess those organs in their young 

 state. The distinction which appears in this respect between the 

 larvse and the mature animals, only shows that Quatrefages has 

 established his new genus Malacoceros w ithout sufficient reason, 

 for the want of eyes is the only character by which it is distin- 

 guished from Nerine. 



The difference of habitat appears to be of no consequence, 

 especially as our larva has a tolerably wide distribution. I cer- 

 tainly do not know where Busch observed his specimens ; but 

 J. Miiller states (Arch, fiir Anat. 1854, p. 92) that he has found 

 it at Marseilles. Miiller however only speaks of a very " similar " 

 larva, but, apparently, merely because his specimen wanted the 

 long spines of the first segment. It is with reason that J. Miiller 

 also points out the great similarity that exists between the 

 large sj)ine-like bristles of our Nerine larv?e and the strong 

 bristles of the singular Mitraria. I think that this circum- 

 stance may even justify us in supposing that the tufts of bristles 

 in Mitraria arc also mere larval organs. At any rate it seems 

 to me that Mitroria is scarcely anything but the larva of an 

 Annelide with a provisional apparatus of bristles, as indeed 

 Miiller himself has lately indicated [loc. cit. sup.). It is much 

 to be regretted that Quatrefages could not follow the develop- 

 ment of his HermellaAai-yx for a longer period ; we should then 

 probably have obtained further data for the elucidation of the 

 species of Mitrarice. I will not lu)wever assert directly that the 

 Mitrarm are actually larvce of Hermella-, for 1 hat would be going 

 too far with our jiresent knowledge, although the elongated capi- 

 tate bristles, which J, Miiller has described, together with the 

 ordinary spines, in the species observed by him, remind one un- 

 mistakeably of the forms of the peculiar paleje of the Hermella. 



As regards Mcsotrocha and its metamorjihosis into Cha>to- 

 jdcrus, M. Miiller has given his interesting observations upo)i 

 this subject in full in the Archiv fiir Anat. und Physiol. 1855, 

 p. 1. In connexion therewith M. Miiller also mentions the 

 larva discovered by Busch, the relations of which to Chcetoptcrus 

 pcrgumentaceiis I have endeavoured to prove, and expresses his 

 o])inion that it does not belong to the genus Clue t opt ems itself, 



