1885.] On the Larval Forms of Spirorbis borealis. 249 
scribed differ in some particulars from those of “ S. spirillum 
Gould (non Pagenst.; an Lam. ?),” described by A. Agassiz (Ann. 
Lyc. Nat. Hist., vit, pp. 318-323). They differ evén more widely 
from the young of S. spirillum described by Pagenstecher (Zeit. f. 
Wiss. Zool., x11). I regard my larvz of the same species as that 
described as S. spirillum by Augustus A. Gould (Report on the 
Invertebrata of Massachusetts [first edition], Boston, 1841). 
A. Agassiz says of S. spirillum (op cit., p. 318): “The eggs, of 
a dark reddish-brown color, are found in strings formed of two 
rows (fig. 18), either on each side of the alimentary canal in the 
anterior part of the body, where in the adult we find a consider- 
able space free of bristles (as in fig. 25), or else when the strings 
have been laid they are found on the sides of the body, between 
it and the limestone tube, and here the young undergo their 
transformations.” Later he says: “The young are quite ad- 
vanced within the body of the parent previous to the transfer of 
the egg-sacs to the cavity of the tube where they complete the 
greater part of their growth.” In his figure 18 the larve in the 
strings have already well formed eye spots. I have never been 
able to observe these larvæ in stages of development in the body 
of the parent, but have found many specimens of eggs outside 
the walls of the body of the adult, which were in the last stages» 
of segmentation and therefore much younger than those) figured 
by him (fig. 18). 
In the first or youngest stage after segmentation a larval condi- 
¿tion was observed in which the embryo almost wholly fills the 
egg capsule and presents very little differentiation in different 
regions. This embryo is of an oblong shape and is girt equato- 
rially by aring of cilia. On one side just below this ciliated belt 
the wall of the embryo is flattened. The body is opaque, has a 
dark brown or reddish color, and is destitute of eye spots. 
In the next oldest stage (Fig. 1), which is very similar to the 
young of the genus Pileolaria, figured by Salensky (Etude sur le 
Developpement des Annelides, Pl. 1v, Fig. 7), we have a central, 
opaque yolk-mass surrounded by a more transparent layer of cells 
which is thickest on the same side as the flattening noticed in the 
walls of the larva in a previous stage, a pair of eye spots and a cres- 
centic-shaped body, which is probably lens-shaped when seen in 
another view, lying between the outer layer of the embryo and 
its inner cell contents. The external layer I have followed Salen- 
sky in regarding as the epiblast and the thin intermediate layer the 
