400 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, Chwtodermat- 

 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 un&er stones on the shores of Norway, but has also been 

 dredged in deep water off the coast. The Neomeniidce comprise the genera 

 Ncomenia, Proneomenia, Lepidomenia, 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, 

 common tooth-shell, Dentalium vulgare (nat. size). consisting of diatoms and f oraminifera. 



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 Dentalium the end has several notches, 



