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MOLLUSCS. 
regard to the disposition of the nervous system. The respiratory organs, when 
present, are terminal and placed within the anal cloaca. In most of the forms the 
foot is reduced to a mere longitudinal groove, and does not appear adapted for 
a locomotive organ. The number of known species is limited. They occur at 
moderate to abyssal depths in the Arctic and Atlantic Oceans, from the Barents Sea 
to the coast of Spain. Two families constitute this order, namely, Chcetodermat- 
idce and Neomeniidce. The former contains but a single genus, Chcetoderma. This 
degraded mollusc, at one time placed among the Gephyrean worms, has an elongate 
worm-like form, with an inflation at both ends. The mouth is terminal, and armed 
with only a single tooth—a poor representative of the molluscan radula. The pedal 
groove is wanting, and the sexes are separate. The only known species—about an 
inch in length—occurs under stones on the shores of Norway, but has also been 
dredged in deep water off the coast. The Neomeniidce comprise the genera 
Neomenia, Proneomenia, Lepidomenici, Ismenia, Paramenia, and Dondersia. 
Neomenia, which has been found off the west of Scotland, ranges from Scandinavia 
to the Mediterranean, and is the best known. N. carinata is about an inch long, 
rather compressed laterally, curved longitudinally, with the back keeled and the 
ventral side with a narrow foot-groove, extending the greater part of its length. 
The mouth is unarmed with a radula, and the sexes are united in each individual. 
The Tooth-Shells, —Class Scaphopoda. 
Everybody knows the tooth-shells, resembling in miniature the elephant’s 
tusks, and often found on the sandy shores of England. They are scientifically 
known as Dentaliidce, and in former times were associated with the marine worms, 
their shells bearing a strong resemblance to the tubes of certain annelids. They 
are more or less elongate, nearly always slightly curved, and are bisexual. The 
head is rudimentary, and in this respect the scaphopods resemble the bivalves. 
They have no tentacles, eyes, or heart, and the organs of respiration and circulation 
are rudimentary. At the anterior end 
of the animal is situated the foot, 
which is not a creeping disc, but 
adapted, like that of some bivalves, 
for burrowing in sand and mud, in 
which they live and obtain their food, 
consisting of diatoms and foraminifera. 
They are said to capture these minute 
organisms by means of a number of long contractile filaments with expanded 
extremities ( tentacula or captacula ) which are situated near the mouth, which is 
armed with a radula, and surrounded by labial palpi. The shell is cylindrical, 
usually somewhat tapering posteriorly, open at both ends, and generally white. A 
few species, however, are of a greenish tint, and others are pinkish. They are 
smooth, longitudinally striated and ridged, mostly circular in section, but a few are 
angular, compressed, and otherwise irregular as regards form. Some are simple at 
the narrow end; but others exhibit a more or less elongate notch or slit on the 
ventral or convex side. In some species of Dentalinm the end has several notches. 
common tooth-shell, Dentalium vulcjare (nat. size). 
