1904. ] NATURAL SCIENCES OF PHILADELPHIA. 389 
of the foot is derived from some of these cells’. Robert. describes a 
similar condition for Trochus. In Fiona not only the median portion 
but also much of the lateral area certainly comes from the third quartet. 
The foot here does not arise as a paired swelling as in Patella (Patten), 
Fulgar (MeMurrich) and Trochus (Robert), but shows from the first 
a median protuberance which increases in size and later becomes 
broadened and flattened (figs. 103, 108, 110). Its upper surface is 
covered with numerous cells, but they are not arranged to form a 
conspicuous cell-plate as in Crepidula. Large cells mark its lower 
surface and they soon begin to secrete the operculum. 
Larval Musculature. 
It is particularly unfortunate that for a study of the muscles of the 
velum no living material has been available, as without this many 
points of interest must of necessity be lost. When the veliger breaks 
from its capsule it presents an appearance shown in figs. 109, 110, 
though it should be remembered that in fixed material, from which 
the drawings were made, the muscles must be much contracted. The 
whole posterior region is swollen into a huge transparent vesicle, at the 
anterior end of which lies the contorted alimentary canal. In dotted 
outline is represented the probable position of the cuticular-like shell 
before shrinkage. In a larva of such age one of the most characteristic 
features is a large dorsal retractor muscle, which has its posterior point 
of attachment well to the left of the dorsal side of the posterior vesicle. 
It runs forward and branches just before reaching the liver lobe, its 
two anterior ends becoming attached to the alimentary canal and the 
body wall in the region of the cesophagus. In structure it is composed 
of large spindle-shaped interlacing cells, which are flattened dorso-ven- 
trally, giving the muscle a band-like form. In function this muscle 
doubtless acts as a retractor for the anterior and particularly the upper 
portion of the cephalic region. A dorsal view of the same veliger 
shows two lateral muscles, the right and left retractors of the foot, 
which arise about midway back on the sides of the posterior vesicle 
and extend forward through the lower part of the neck region, to end 
in branching fibers in the foot. That of the right side is larger than 
the left, and in earlier stages (figs. 105, 106) is much thicker than later 
and relatively larger. In figs. 105 and 106 is shown a small muscle 
(V1.R.) extending from the dorsal neck region to the velar folds where 
it branches greatly. Other similar retractor muscles of the velar 
lobes extend from the walls of the alimentary canal and the body wall 
