eS 
ae, 
1885.] On the Larval Forms of Spirorbis borealis, 253 
covered with cilia. Midway between this last and the posterior 
extremity of the body is a second prominent tuft, and the very 
end has a third. Besides these three ciliated regions the whole 
ventral surface of the posterior part of the body of the larva is 
clothed with small cilia. 
If the cephalic region of the last mentioned larva be looked at 
from one side (Fig. 12), it will be noticed that on the left hand 
side of it, snugly approximated to the dorsal walls of the head, 
there is a thin triangular plate which has begun to push itself up 
from the region just in advance of the ferrule-like structure upon 
which the ring of larger cilia is borne. This structure which at 
first grows out from the left hand side of the neck, and is un- 
paired is the future operculum.! At the time of its origin it is 
small, and in all stages unpaired, while later in the growth of the 
worm it assumes a considerable size. 
The first appendage to form on the right hand side (Fig. 11) 
of the larva is a small tentacle, short, stout and club-shaped. I 
do not find another similar tentacle on the left hand side, but it 
it exists it may be hidden by the operculum. The better inter- 
pretation, however, is, that the operculum where it originates has 
the form of a simple tentacle with which it is strictly homolo- 
gous ; while the size of the operculum as it grows increases so 
greatly that it far outmeasures that of the right hand tentacle. 
My observations do not lead me to think that the tentacles form 
alternately on the sides of the head of this species of Spirorbis. 
Fig. 13 represents a slightly older larva seen from the dorsal 
side, in which the segmentation of the posterior body region is 
more sharply defined, and in which also the operculum (ef) and 
the right hand tentacle (ża) are well developed. In Fig. 14 we 
have the same larva much older, shown from the ventral side, 
where the operculum is represented as expanded on the left hand 
side of the head. In most particulars this larva closely resem- 
bles the free larva (Fig. 10). 
In a stage which is older than the last the larva has passed 
into a condition in which not only has the operculum assumed a 
considerable size, but also several small appendages are found on 
the head, while of the organs of the head which have disap- 
odd opercular tentacle covering in figure 21 the right tentacle.” 
was the first cephalic appendage which was observed in the larvæ which I studied. 
