1885.] On the Larval Forms of Spirorbis borealis. 251 
Fig. 9 is the youngest stage of development of Spirorbis in 
which lateral spines were observed, and in it there are two pairs, 
a single spine on each side in each pair. This character is also 
recorded in one of the larval stages observed by A. Agassiz, he 
says: “The bristles make their appearance in figure 21, where 
we find two of the three bundles of the collar-like projection of 
the anterior extremity always distinctly marked in such young 
embryos.” In figs. 4 and 5, Pl. xxx1x, in Pagenstecher’s account, 
it appears in the youngest stage that the spines are represented 
by a single pair. . 
The larva in my plate (x1, Fig. 8) is represented as divided into 
three marked regions, which from now on will be known as the 
anterior or cephalic, the middle covered on its ventral side by a 
much larger growth of the collar and a smaller posterior region. 
The first and second of these divisions are separated by a ring of 
cilia, the second and posterior by the posterior border of the col- 
lar. The prominent lateral ocellus lies on the ventral side of the 
larva and has a bright red color. The whole body of the em- 
bryo is reddish, while the external surface of the collar as 
well as the ventral region of the posterior part of the embryo is 
covered with small cilia. On the walls of the ventral region just 
below: the collar there arises a brick-red projection. I have 
homologized this projection with the “glandes tubipares” de- 
scribed by Salensky in the young of the genus Pileolaria. In 
this latter genus, however, these glands are arrayed laterally in- 
stead of medially and vertrally. In Spirorbis as in Pileolaria I 
find three ciliated regions which we may follow Salensky in des- 
ignating: 1. Couronne ciliaire. 2. Couronne ciliaire abdominale. 
3. Couronne ciliaire anale. 
The interior of the larva on the dorsal side is occupied by a 
brownish body which is in part the unabsorbed yolk mass. 
Through all the stages of growth thus far traced the embryo is 
still included in its egg capsule. It was observed to fret continu- 
ally against its envelope and with its spines it constantly presses 
upon the walls of the same. The motion of these spines shows 
at once that the egg capsule has flexible walls yielding easily to 
such pressure. In the next stage (Fig. 10) the larva has become 
free from the capsule, swimming about in the water with consider- 
able activity. Judging from A. Agassiz’s statement in regard to the 
amount of the development of the side branches of the tentacles, 
