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. 



FIG. 184. 



Embryo of Dentalium, with six 

 micromeres or ectodermic cells 

 and a single macronwre or endo- 

 dermic cell, ma, inacromere ; mi, 

 micromere. (After Kowalewsky.) 



FIG. 185. 



Larva of Dentalium, aged one 

 and a half day ; ventral aspect. 

 I, foot; II, mantle; III, velum 

 forming a sort of test. (After 

 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, etc. 



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. 



FAMILY 1. DENTALIIDAE, 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 



