322 On the Young Stages of a few Annelids. 



The first tentacle appears on the right, Fig. 20 t x , next conies 

 the corresponding tentacle of the left, and only later, Fig. 21. 

 the rudiment of the odd opercular tentacle (7 , Fig. 22), cover- 

 ing in Figure 21 the right tentacle. The bristles make their ap- 

 pearance in figure 21, where we find two of the three bundles 

 of the collar-like projection of the anterior extremity, always 

 distinctly marked in such young embryos. In the next stage 

 the collar is more prominent, and an additional bristle is found 

 representing the third bundle of the collar (Fig. 22). The pos- 

 terior 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 t 2 , tf 3 (the opercular tentacle always 

 retaining its peculiar shape). 



In subsequent stages, Fig. 23, 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 Figure 24 ; the opercular tentacle has be- 

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

 Figure 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 Figure 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 * 



* Muller Fritz. Fur Darwin. Leipzig. 1864. 



