256 On the Larval Forms of Spirorbis borealis. (March, 
The collar, which when the head is extended from the case, is 
reflexed over the edge of the shell, is clothed with minute cilia. 
As in previous larve, drawings are very difficult to make on ac- 
count of the quick motion in the retraction of the head, in this 
the difficulties are even greater. A camera drawing of the case 
is, however, very easy to obtain from its more solid nature. The 
soft parts were taken from a specimen which was dead, but not 
distorted by the conservative fluids in which it was preserved, 
On the body of the worm three pairs of spines, which are hook- 
shaped at their extremities and connected with the body walls by 
strong muscles, were observed. These spines are placed as in 
early stages upon the anterior body region, and are very promi- 
nent. The posterior body region is destitute of spines in this 
stage. One of the most conspicuous structures in the body of 
the worm is a large oblong mass, of reddish color, easily seen 
through the transparent walls of the case which encloses the 
worm. The size of the worm is 2™" measured from one side of 
the coiled case to the opposite side. 
EXPLANATION OF THE FIGURES. 
' cap, capsule in which the embryo is enclosed. 
6s, covering in which the ‘‘strings ” of ova are found. 
Éj, projections which later grow together and form the collar. 
r, posterior body region, 
za, tentacle. 
PLATE XI. 
Fic. 1.—The terminal egg of a string in its capsule pies segmentation and formation 
of layers and cilia. Larva in capsule and chai 
(Figs. 2-4, the same embryo still in eile older than the last.) 
2.—Lateral view. 
3.—From posterior pole. Larva in capsule iid chain. 
4.—From ventral side. ; 
(Figs. 5-7, still older larva.) 
5.—Lateral view. 
6.—Ventral view, 
7-—View from anterior pole. 
8. eres view of a larva slightly more mature than the last. 
9.—Old from ventral surface just before escape from its capsule, Ter- 
minal larva in the chain, aig a 
