3370 THE MOLLUSKS OR SHELLFISH 



They are related to the chitons by certain points in their anatomy, especially with 

 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 

 idee and Neomeniidce. The former contains but a single genus, Chcetoderma. This 

 degraded mollusk, 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 molluskan 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, 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 TOOTHSHELLS Class Scaphopoda 



Everybody knows the toothshells, 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 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 circula- 

 tion 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 TOOTHSHBU,, Dentalium vulgare. cons i st i n g of diatoms and foraminifera. 

 (Natural size.) m . 



They .are said to capture these minute 



organisms by means of a number of long contractile filaments with expanded 

 extremities (fentacula 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 



