On the Young Stages of a few Annelids. 323 



The principal changes take place almost exclusively in the 

 anterior extremity ; the posterior part of the body does not 

 lengthen until the collar and tentacles may be said to be fully 

 developed, and although we find papillae on the sides ot 

 the posterior part of the body, similar to those forming the 

 single loop of the collar of the adult, as well as the peculiar 

 scythe-shaped bristles of each ring ; yet the young Spirorbis 

 has, up to this time, passed through no phase of growth during 

 which the increase of the posterior part was in the least to be 

 compared with the changes of the anterior extremity. In 

 nearly all other Annelids we find the posterior extremity play- 

 ing a much more prominent part in determining the shape ot 

 the young worm. This is undoubtedly due to the shortness of 

 their nomadic life ; and though capable of active movements 

 during that period by means of the collar, their freedom soon 

 comes to an end, and they complete their development after 

 having assumed the habits of the adult. 



Polydora. Bosg (Leucodora Johnst.). 



Claparede having given in his Beobachtungen a very com- 

 plete history of the development of what he calls Leucodora 

 ciliata, the following observations would be superfluous as far as 

 they relate to new phases in Polydora, but may be useful in 

 clearing up the confusion existing concerning the identity of 

 Leucodora Johnst. and Polydora Bosc. Quatrefages, in his Syn- 

 optic Table,* has introduced these two genera as distinct, and 

 separates them on account of the remarkable structure of the 

 bristles of the fifth ring in Polydora, which he says is 

 not to be found in Leucodora ; this must evidently be a 

 mistake, as Johnston's figuref certainly possesses the peculiar 

 bristles of the fifth ring as maintained by Claparede in his Beo- 



* Quatrefages A. de. Note sur la classification des Annelides; in Comptes Ren- 

 dus. 27 Mars. 1865. 



f Johnston Geo. Miscellania Zoologica ; in Mag. Zool. Bot. 1838. ii. p. 66. 



