On the Young Stages of a few Annelids. 321 



fifth ring. The description given by Stimpson* of the genus 

 Lmnara agrees so well with some of the stages of Terebella, 

 that I am inclined to consider it only as an embryonic condition 

 of some allied Terebella. Long after the stage here fig- 

 ured, even when the branchiee have become quite well deve- 

 loped, it is very common to fish up with the dip-net these young 

 Terebellas, which are capable of a certain amount of motion by 

 the contortions of the tentacles and body. They build their 

 cases very late, and frequently leave them to climb about on 

 eel-grass, piles, etc., making considerable progress with the aid 

 of their tentacles, by which they drag themselves along. 



Pagenstecher has invariably represented the tentacles of the 

 anterior extremity of Spirorbis as developing symmetrically 

 and in pairs. This is not the case in our species, where they 

 are formed very differently from what has been thus far observ- 

 ed in this family. We have between these two modes of growth 

 a difference similar to that existing between Terebella nebulosa 

 and T. eo?ichilcga, where in one case the tentacles appear 

 successively, while in the other they are formed in pairs. The 

 oldest tentacles of our Spirorbis are formed on the outside, new 

 tentacles appearing successively singly nearer the median line 

 on alternate sides, and not in pairs; the corresponding tentacles 

 on each side of the middle line being of very different lengths. 

 This want of symmetry is readily seen in the youngest speci- 

 mens figured, Figs. 20, 21, 22 ; and though it is more difficult 

 to trace this in older stages (Fig. 25), the presence of the simple 

 opercular tentacle always introduces a prominent asymmetri- 

 cal element, soon lost in the more advanced stages of the de- 

 velopment of Terebella. The two eyes are quite prominent, and 

 can generally be traced in the adult, although they are not as 

 striking as in the younger stages ; the ocular spots are always 

 limited to two, and we find at no time either a ring or clusters 

 of eye-specks. 



* Stimpsom W. Marine Invertebrates of Grand Manan. 1853. p. 30. 



