334 On the Young Stages of a few Annelids. 



from cylindrical, the head is quite rectangular, with rounded 

 corners, and nearly as large as the rest of the embryo. The 

 shield bulges out nearly to the anal extremity, where it sud- 

 denly terminates, and leaves exposed the small rings pre- 

 ceding the anal ring ; this terminal ring has not the promi- 

 nence so characteristic of other Annelid embryos. On the 

 lower side, immediately behind the vibratile ring, we find a 

 slight swelling towards which the digestive cavity point;?, and 

 where the mouth eventually is formed, Fig. 50. while the 

 rudimentary paddles of the rings are plainly visible along the 

 sides. The motion of these larvae, as can be readily ima- 

 gined from the size of the cilia, is exceedingly rapid, and 

 though occasionally at rest for a short time, their gyrations 

 are most unfortunate for careful observations. 



In subsequent stages we find that the posterior part, as in 

 nearly all young Annelids, undergoes the greatest changes. 

 The head has retained its shape, and its appendages have not 

 enlarged ; the shield and the body have both lengthened, Fig. 47, 

 lower fig. The rings of the young worm are quite distinct, the 

 broad flap (the future paddle) is more prominent ; small cirri are 

 developed, from which push out a single rather stiff bristle, and 

 two smaller jointed ones. Two small anal cirri have grown at 

 the extremity of the anal ring ; on each side of the anterior 

 end of the shield we find two long tentacles, of different size, 

 the first pair of tentacles of the adult, the dorsal one being the 

 longest. When viewed in profile, Fig. 50, the mouth is seen 

 as a large rectangular opening, Fig. 49, placed within the 

 edge of the shield, which extends on the lower as well as the 

 upper surface, though only as a narrow band in the central 

 side without covering any of the rings. The body behind the 

 shield is fully as prominent as the remainder of the embryo, 

 and the broad oars of the rings show undoubted!} 7 that we have 

 to deal with a Phyllodoce. A second pair of small anterior 

 tentacles is budding at the base of the first. Though the 

 digestive cavity opens externally at the mouth and anus, there 



