M. R. Leuckart on the Young States of some Annelides. 269 



the root of the caudal appendage that it possesses the structure 

 described by Krohn. Beyond that point it narrows suddenly to 

 about a fourth of its previous width, and then proceeds in the 

 form of a thin straight tube to the anus. The only vessel 

 observed was the pulsating dorsal vessel. Lateral glands are 

 deficient. 



The changes through which the worm has to pass before it 

 returns to the perfect form of its parent, are of two kinds. In 

 the first place the neck will gradually shorten and disappear, 

 with the exception of its segmental appendages*, so that the 

 head will appear to sit immediately upon the first pair of setose 

 feet. The segments of the caudal appendage then gradually in- 

 crease in number and become developed so completely, that at 

 last the difference between them and those of the middle of the 

 body is lost. The temporary existence of this distinction perhaps 

 indicates, that a state of rest intervenes between the formation 

 of the middle region and that of the posterior segments, during 

 which the plastic processes are entirely confined to the develop- 

 ment of the parts already formed. The formation of the poste- 

 rior segments of the body as here exhibited, presents so remark- 

 able a resemblance to the reproduction of the body after muti- 

 lation, that I should not have regarded our worm as a normal 

 stage in the development of Ale. Raynaudii, if its small size, the 

 structure of the neck, and especially the presence of a range of 

 cilia behind the head, had not sufficiently indicated that it was 

 an immature and undeveloped creature. 



Supplementary Notice. 



Long after I had sent the preceding sheets to the press, the 

 * Magasin de Zoologie^ for the year 18Jt3, with a paper by Qua- 

 trefages upon some new Annelides from Normandy, which had 

 hitherto escaped my notice (and which is not even mentioned in 

 Grube's Monograph), came accidentally into my hands. At pi. 3. 

 fig. 7. of this work, a Spioid Annelide is figured under the name 

 of Malacoceros longirostris, which from its appearance and the de- 

 scription annexed may probably be the mature state of the spinous 

 larva observed by Busch and myself. The formation of the 

 cephalic tubercle especially is perfectly similar, and it is from 



* Krohu only mentions four tentacular cirri on each side in A. Ray- 

 naudii, but leaves it uncertain whether these re|)rcsent the appendages of 

 two or four segments. This point may )ierha])s be settled by my observa- 

 tions. On the other hand it must still remain undecided, whether th 

 small foremost cirnis gradually disap}iears or has been overlooked by 

 Krohn. 



