218 Mr. A. Agassiz on the Young Stages of a few Annelids. 



tenor extremity has lengthened, the anal cirri have nearly dis- 

 appeared, and a couple of very indistinct articulations can be 

 traced behind the collar. There are also two additional tenta- 

 cles placed between the first pair, which readily show in what 

 order they have appeared (/g, ^3), the opercular tentacle always 

 retaining its peculiar shape. 



In subsequent stages (fig. 33) the posterior extremity has 

 lengthened but slightly. There are along the side of the poste- 

 rior part of the body a couple of bristles similar to those of the 

 adult; we can trace the first stage of the bifurcation of the four 

 tentacles at their extremity, rendering the age of the tentacles 

 more apparent, as in fig. 24 ; the opercular tentacle has be- 

 come more funnel-shaped. At about the period represented in 

 fig. 23, the young Spirorbis escapes from the egg, and leads 

 a short nomadic life ; it soon attaches itself, and in less than 

 twelve hours after hatching has built its limestone tube, in 

 which it henceforth lives : subsequent observations can only be 

 made by crushing the shell, as it is not transparent enough 

 to show the young worm. The tentacles take a rapid develop- 

 ment; and in fig. 25 we have a small Spirorbis having only 

 nine rings, with tentacles nearly as branching as those of the 

 adult, and a well-formed operculum, which with advancing age 

 loses all trace of its former tentacular nature. The tentacular 

 nature of the operculum in this family has also been observed 

 by Fritz Miiller ■^. 



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 of 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 com- 

 pared with the changes of the anterior extremity. In nearly 

 all other Annelids we find the posterior extremity playing a 

 much more prominent part in determining the shape of 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. 



[To be continued.] 



* Fiir Darwin. Leipzig, 1 64. 



