346 Mr. A. Agasslz on the Young Stages 



following the seventh. The bunches of long bristles of the 

 four anterior rings are gradually losing their prominence, be- 

 coming less numerous in the present stage, and are replaced 

 by bristles similar to those of the other bunch. At this period 

 the number of rings does not increase rapidly; the principal 

 changes are confined to the growth of the dorsal cirri and of the 

 tentacles, as well as to changes in the pigment-spots. When 

 examined in somewhat more advanced stages (fig. 29), from the 

 lower side, we still have the pigment-spots prominent and well 

 marked; the only change consists in the lengthening of the 

 tentacles and the dorsal cirri, which are both fringed on their 

 anterior edge by vibratile cilia. 



On examining a young Polydora from the dorsal side, some- 

 what more advanced (fig. 30), still having the same number of 

 segments in front of the anal ring, w^e find the four lines of 

 brilliant star-shaped pigment-spots diminished to four rows of 

 dots; the body has grown somewhat opaque and assumed a 

 reddish tinge, especially along the alimentary canal. At about 

 this period, also, the young begin to build their case, secreting 

 a copious viscid fluid (from the glands at the base of the dorsal 

 cirri ?), to which particles of sand and mud become attached as 

 they creep along the bottom ; although they frequently leave 

 this case, they no longer possess the great power of locomotion 

 of the young larvje (figs. 26-28), which moved about rapidly by 

 means of their rows of vibratile cilia and bunches of long 

 bristles. The pigment-spots of the head have disappeared, ex- 

 cept four prominent eye-specks, — the same number as found in 

 the adult, in which, however, they are far less conspicuous than 

 in this stage of the young. 



When the young worm has already thirty-five rings, there 

 have been no changes of any consequence besides the further 

 lengthening of the dorsal cirri and the increase of the rudimen- 

 tary cirrus at their base, which can first be traced in fig. 29 ; 

 two small tentacles have been formed at the anterior part of the 

 head (Pl.VI. fig. 31). The number of bristles of the fifth ring has 

 increased to five, and the number of glands in each envelope to 

 eight or ten. In nearly full-grown specimens, when seen from 

 below, these glands are particularly prominent (fig. 32), as well 

 as the six hook-shaped bristles at the base of the short cirrus. 

 The black pigment-dots have all disappeared, and the worm is 

 gradually assuming a darker tinge; the fifth ring has increased 

 in width, the larger of its bristles assuming the shape they have 

 in the adult, as in fig. 37, with a slight notch at their swollen 

 exterior extremity. The simple hook-shaped bristles of the 

 seventh and succeeding rings have developed a slight process 

 on the convex side, with a stifi" bristle (fig. 38) extending from 



