250 On the Larval Forms of Spirorbis borealis.  [ March, 
mesoblast. The ring of cilia is seen in profile on each side above 
the equator of the embryo. 
One of the best eggs to use in following the changes has been 
found to be the last of the chain, since it is always isolated, on 
one side at least, from the rest. We often find strings of ova with 
the eggs in a single row. These eggs have always been found 
well suited for study on account of this simple arrangement. The 
members of a chain can also be separated from each other with- 
out injury. 
In the next oldest larva to that just mentioned (Figs. 2, as 
we find the indentation on the flattened side of the embryo still 
more pronounced than before, as shown in Fig. 2, which repre- 
sents the larva as seen from one side. If this larva is seen from 
the posterior pole (Fig. 3), we notice two prominent protuber- 
ances which impart to it as seen in this way an irregular triangu- 
lar shape. From the ventral side (Fig. 4), upon which the pro- 
tuberances lie, these appendages appear as small lateral projec- 
tions (#7) on each side. The embryo fills almost the whole inte- 
rior of the capsule in which it is now confined, and the protuber- 
ances lie just below the ring of vibratile cilia. From this stage it 
seems that the collar which is later found on the ventral side of 
the larva originates as two projections, one on each side. Ina 
larva somewhat older than the last (Figs. 5, 6, 7), the collar (co?) 
has formed by the union of the two projections, and has grown 
somewhat downward over the ventral side of the posterior region 
of the body. Fig. 7 shows the same embryo as seen from the an- 
terior pole, and Fig. 6 the same from the ventral side. The dor- 
sal surface of the larva is more curved than in younger em- 
bryos. 
In Fig. 9 the larva is still enclosed in its capsule, and is repre- 
sented from the ventral region, while the collar is still more de- 
veloped, and two pairs of single spines were observed in the 
region partially covered by the collar.’ 
: 1 Pagenstecher describes and figures (of. cit., Pl. xxxix, Fig. 6) a first pair of 
_ spines consisting of three on each side at the base of the collar which I have not 
found in my larve. He says, ‘-An der Wurzel des Kragens sprosst das erste Paar 
von Borstenbiindeln hervor, vorläufig mit je drei Borsten.” I do not find these rep- 
resented in the young of Spirorbis borealis, and A. Agassiz neither mentions nor fig- 
ures them in his Spirorbis. My observations 5. not agree with those of Pagenstecher 
when he says, “ Die erste Spur der Tentacle zeigt sich in Form von drei Höckern 
~ jederseits auf dem Kopflappen.” I have also been unable to find in my species the 
2 ae RY o tin geben Fleck,” which he describes ‘‘ neben dem Magen 
