130 BULLETIN OF THE 
which was thought to be the infolding to form the proctodeum of the 
last stages of the gastrula. The epiblastic cells, although becoming 
thinner by the growth of the mesoblast, are still large and prominent, 
while the hypoblastic are still cylindrical in shape. Both are ciliated. 
The former layer furnishes the motor cilia of the body; the latter the 
ciliated lining of the stomach. 
Immediately after the first stage in the formation of the calcareous 
spicules in the larva, which from now on ceases to be a gastrula, the 
anterior lobe, al, and the two prominences which form later the poste- 
rior arms, pr, begin to push out, and the region in which they form has 
a resemblance to the three legs of a tripod. The larva when seen from 
the oral or ventral side, Pl. V. fig. 5, has on each side of the mouth, in 
a plane in advance of this opening, a small prominence, pr, thickly pig- 
mented, especially on its distal end, into which extends a rod from the 
stellate calcareous rod of each side. In the interval between these rods 
a large undivided lobe, o/, bearing the mouth, appears. This undivided 
lobe is the anterior or oral lobe, and on its ventral surface is a circular 
ciliated opening, o7, the mouth. As this lobe grows, the mouth opening 
is carried up with it on its side. When seen from one side, so that the 
length of the two prominences, posterior arms, and oral lobe can be 
compared, it will be found that the anterior lobe is more prominent 
than the two posterior arms. In this stage the body of the larva is 
nearly spherical in form, and as it rests on a tripod formed of the two 
posterior arms and the single anterior lobe, the intervals between these 
prominences are easily seen. The anal pole of the larva is pigmented, 
and filled with numerous ameeboid and mesoblastic cells. When seen 
from the oral pole, we notice that the ventro-dorsal diameter is about 
the same as the lateral. The mesoblast is much thicker than either 
the hypoblast or the epiblast. A broad band of cilia surrounds the 
mouth. 
In a pluteus two days old, Pl. V. fig. 6, raised from the egg, we have 
the two posterior rods still more developed than before, while the ante- 
rior lobe is still undivided. Seen from the ventral side the distal ends 
of the two posterior rods diverge in a V-shape, while the posterior face 
of the anterior lobe appears rectangular. The opening of mouth and 
anus are well seen. 
In the interior of the pluteus we notice that the calcareous rods 
which support the posterior arms are double, and have not joined to 
become latticed. From a point in the body of the pluteus on a level 
with the anus these rods join the system of rods of the body. One 
