394 PROCEEDINGS OF THE ACADEMY OF [April, 
present. The stomodzum later forms at the point where the blasto- 
pore closed. 
The shell-gland at first forms a deep invagination, which later opens 
out and covers the posterior end of the veliger with a cap of large 
cells which soon begin to secrete the shell. From the first the shell 
is slightly shifted toward the left, and this asymmetry becomes more 
marked with continued growth. With the enlargement of the shell 
a conspicuous posterior vesicle results. 
The foot arises as an unpaired swelling below the stomodzum. Its 
under surface later secretes an operculum. 
The first velar row is formed from the anterior trochoblasts (A and 
B quadrants), the tips of the anterior arm of the cross, and possibly 
from other cells of the first quartet in this region. The second velar 
row is derived from underlying cells of the second quartet. A post- 
oral velar area is but slightly marked. In later development the velum 
becomes bilobed and broadly expanded. 
A prominent head vesicle is not present in the older veligers, and 
with this may be correlated the development of a large posterior vesicle. 
No apical sense-organ has been found, nor are distinctly marked apical 
plates present. The cerebral ganglia appear in the angles between 
the anterior and lateral arms of the cross. Otocysts are formed by 
invaginations of the ectoderm upon the sides of the foot, and pedal 
ganglia appear closely associated with them. The eyes are late in 
appearing and are intimately connected with the rudiments of the 
cerebral ganglia. 
The anal kidney of the larva is derived from the ectoderm, coming 
from 3c" and associated cells. With the torsion of the larva this 
group is shifted farther to the right, and eventually lies well up on the 
right side of the veliger above the anal opening. Primitive ex- 
cretory cells are also found lying in the body cavity laterally behind 
the velum. 
The enteron is formed by invagination of the entomeres, which at 
first form an elongated sac; with the evagination of the shell-gland 
this becomes rounded. The liver is derived from large yolk-ladened 
cells lying at the anterior end of the enteron, and later the rudiment 
of this organ becomes turned toward the left side. Torsion of the 
enteron results from lengthening of the left side and is caused by 
increased growth of that region. The intestine is at first a solid thick 
cell-strand and is composed largely of entoblasts from 4d; it later 
elongates and acquires a lumen. 
