THE SCAPHOPODA 203 
The pedal prominence arises on the ventral surface and grows 
forward ; after the disappearance of the larval velum the foot is used 
for creeping. The cerebral ganglia arise as two deep symmetrical 
ectodermic invaginations in the velar area. The otocysts are formed 
as invaginations of the surface of the foot, and the pedal ganglia 
originate after the otocysts from thickenings of the ectoderm. 
The endodermic cavity gives rise to the stomach and _ intes- 
tine. The liver is developed in connection with the wall of the 
stomach. The anal opening is not formed till a very late period. 
At the end of five or six days the velum atrophies, the young 
animal ceases to swim and begins to crawl along the sea bottom. 
% 
Fic. 184. Fic. 185. 
Embryo of Dentalium, with six Larva of Dentalium, aged one 
micromeres or ectodermic cells and a half day; ventral aspect. 
and a single macromere or endo- I, foot; Il, mantle; III, velum 
dermic cell. ma, macromere ; mi, forming a sort of test. (After 
micromere. (After Kowalewsky.) Kowalewsky.) ‘ 
IV. BIoNoMIcS AND DISTRIBUTION. 
The Scaphopoda are marine burrowing molluscs, and as a rule 
only allow the posterior extremity to project from the sand in 
which they hide themselves. They feed on the lowest organisms, 
Diatomacea, Protozoa, ete. 
There are 150 living and nearly 275 fossil species of Scaphopoda. 
The living forms are distributed throughout all seas from the littoral 
to a depth of 2500 fathoms. The fossil species extend back to the 
middle Silurian, but are most abundant from the Cretaceous onwards. 
V. REVIEW OF THE FAMILIES OF SCAPHOPODA. 
There are two different types in this homogeneous group, but the 
differences between them are not of more than family value. 
Famity 1. Dentanimar, Gray. Foot conical with a laterally ex- 
panded and dorsally interrupted encircling sheath. Shell tubular, curved, 
with the greatest diameter at the anterior aperture, and tapering evenly to 
