M. R. Leuckart on the Young States of some Annelides. 263 



as we know with certainty, are always destitute of such appa- 

 ratus. It is true that Quatrefages professes to have observed 

 some small Annelida with permanent tufts of cilia, but the ani- 

 mals in question are still but imperfectly known, — still less have 

 they been ascertained to be fully developed species. 



If we leave these bristles out of the question, there remains a 

 slender worm, with an eye-bearing cephalic tubercle, and two 

 tentacular cirri, with a simple (not protrusible) unarmed oeso- 

 phagus, slightly developed dorsal cirri, and differently formed 

 simple bristles in the pedal tubercles, which are also but slightly 

 prominent. Of course many changes take place in this animal 

 before it arrives at its perfect development and sexual maturity, 

 but these will hardly be of great importance ; they will probably 

 be confined to the enlargement of the tentacular and dorsal 

 cirri. 



If we consider this proved, there can be little doubt as to the 

 systematic position, or, which is the same thing, the origin of 

 our animal. Our worm is one of the so-called Dorsibranchiata, 

 and indeed belongs to the family of the AricicB^ as circum- 

 scribed by Oersted (Wiegmann^s Archiv, 1844, i. p. 103). The 

 existence of a single pair of tentacular cirri refers it to the group 

 of AricicB NaidimB, and the uniform structure of all the indi- 

 vidual segments of the body places it under the genus Nerine 

 or Spio. Without a knowledge of the mature form, it is difficult 

 to decide to which of these two genera the worm belongs ; this 

 will depend especially upon the subsequent condition of the 

 apical segment, which in Nerine is provided, as is well known, 

 with a sucker-like circle of papillae. 



Of the two genera above mentioned, we know at present in 

 the Mediterranean only a single representative, and that but 

 imperfectly — the Lumbricus cirratulus of Delle Chiaje. At the 

 moment I have unfortunately no opportunity of consulting the 

 Memoir in which this worm is figured (tab. 64. fig. 16), and 

 must therefore leave it undecided whether our larva may be 

 referred to this species. 



The developed Spiones and Nerince live in mud, where they 

 form regular passages and tubes ; the free pelagic life of our larva 

 is therefore subsequently, probably after the loss of the ciha, 

 exchanged for another and very different residence. 



If, however, further proofs of the derivation of our larva 

 from a Spioid Annelide be required, we need only refer to the 



* For comparison of the form of the bristles I may refer here to my de- 

 scription oiLeucodore mutica (Wiegm. Arch. 1849, i. p. 200). With regard 

 to the structure of the oesophagus I may also mention the genus Aonis, 

 which, according to my observations (Wagner's Zoot. ii. p. 307), in this 

 respect agrees most closely with our larva. 



