298 WILSON. [Vor Tx: 
found with a little mass of protoplasm and a nucleus sticking 
to one side, but I could come to no conclusion as to their mode 
of formation. The bow-shaped spicules, mentioned as present 
in the earlier stage, are now found in greater number, but still 
there are only a few of them. In the hollow of the bow there 
is an accumulation of protoplasm with a nucleus, and the indi- 
cations are that the spicule is formed as a superficial secretion 
of this mass of protoplasm. The bow-shaped spicules are almost 
all found in the posterior half of the larva. The same is true 
of the third kind of spicule, the rosettes of embryonic shovels. 
No rosettes are shown in Fig..29, but in Fig. 30 there are 
three shown, and Pl. XVII, Fig. 34, represents such”a rosette 
(seen in section) more highly magnified. The spicules are 
very thin and delicate as well as small, and are not (at this 
time) found separately, but always united in rosettes. The 
rosettes are few in number and are usually found close under 
the ectoderm at the posterior pole. 
4. METAMORPHOSIS. 
The ciliated larva swims freely for a day or two. Asa rule, 
some time during the second day after birth, it sinks to the 
bottom and begins to attach. The first step in the metamor- 
phosis takes place while the larva is still swimming freely 
about. This consists in the flattening of the ectoderm. Pl. XVI, 
Fig. 27, represents a surface view of a larva 36 hours after 
birth, and Fig. 30 a longitudinal section of the same stage. 
On comparing these figures with the corresponding figures 
made from a larva just hatched (Figs. 26 and 29), it will be 
seen that the posterior unpigmented area, or region of flat 
ectoderm cells, has increased in extent at the expense of the 
pigmented area or region of columnar cells. By keeping the 
same larva under observation, it can be seen that the unpig- 
mented area gradually extends forwards. As I have said, the 
process begins while the larva is swimming freely about. It 
continues after the larva has sunk to the bottom. Pl. XVI, Fig. 
28, shows a surface view of a larva in course of attachment. 
In this larva the pigmented region is reduced to a small area at 
