250 Mr. A. Agassiz on the Young Stages 



round the anal extremity. There are two very prominent eyes 

 placed near the anterior extremity^ and two quite rudimentary 

 tentacles. There is as yet no exterior communication from the 

 digestive cavity, which is simply blocked out, occupying little 

 more than two-thirds of the space in front of the vibratile ring 

 and of the large shield extending behind it ; when seen in pro- 

 file (fig. 47, upper figure), the cavity is somewhat retort-shaped, 

 and occupies mainly the dorsal portion of the embryo. Imme- 

 diately behind the shield (fig. 46), we find the small conical 

 body, w^here we can already trace the first indications of the 

 broad paddles of Fhxjllodoce as delicate transverse swelhngs 

 on each side, connected by slight articulating fines. The arti- 

 culations are tolerably distinct when seen in profile (fig. 47, 

 upper figure) ; from this point of view the embryo appears far 

 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 preceding 

 the anal ring; this terminal ring has not the prominence 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 points, 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 ]arv3P, as can be readily imagined from the size of the 

 cilia, is exceedingly rapid ; and though occasionally at rest for a 

 short time, their gyrations are most unfavourable 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 (Hg. 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 stitt^ 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 diff"erent sizes 

 (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 on the ventral 

 one, 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 undoubtedly that we have 



