GASTROPODS— PTEROPODS— BIVAL VES 305 



Quite different from all other gastropods are the chitons (Chitonidce), which 

 have the habits of limpets, but in external appearance look more like huge wood- 

 lice, the shell consisting of a number of movable transverse plates, and the 

 animal having the power of rolling itself up into a ball. Ranging in size from half 

 an -inch to 6 inches, they arc found in all seas, generally near the shore but 

 sometimes at great depths. They are never very abundant, although there are no 

 less than eleven species on the British coasts, the largest of these being Chiton 

 discrepans, in which the length of the shell is about 1^ inches. 



An important group of gastropods characterised by the backward position of 

 the gills have the shell either wanting or more or less completely enveloped in the 

 body. Among the latter section of the group are the so-called bubble-shells, typified 

 by Bulla ampulla of the Atlantic and Indian Oceans. In this species the shell is 

 smoothand globose, marked with brown specklings upon ayellowish ground. Another 

 type is presented by the globe-shell ( Acera bullata) of European seas, which swims 

 by means of the side-lobes which envelop the rounded shell. A second section, in 

 which the shell is small or occasionally absent, includes the miscalled sea-hare 

 (Aplysia depilans) of the Mediterranean, in which the shell is arched and flat and 

 measures no more than a couple of inches, although the length of the entire animal 

 is 9 or 10 inches. The naked-gilled gastropods, which include some seventeen families, 

 have no shells in the adult state and differ in many other respects from the fore- 

 going. One of the most striking forms is Dendronotus arborescens, of the seas of 

 northern Europe, which is covered with such a mass of tentacles as to resemble a 

 moving bunch of seaweed, this structure being intended for its protection. 



Among the pelagic molluscan fauna of the sea — that is to say the 

 Pteropods. «' . . . 



free-swimming forms found on its surface — none are more important 



than the pteropods, so called from the pair of fin-like structures into which the 



lateral portions of the foot have been modified. By their aid these small molluscs, 



which often occur in countless millions, rise at the approach of night from the ocean 



depths to the surface, where they swim about for several hours in search of food. 



When satisfied they again sink to the depths, contracting their fins and withdrawing 



the body into the shell or mantle. Pteropods inhabit every sea, not even excepting 



the Arctic Ocean ; and in many regions the ocean floor is strewn with their 



empty shells for acres. One of the northern forms constitutes a large proportion 



of the food of the Greenland whale. In one group the shell persists throughout life, 



whereas in a second it disappears before maturity is attained. The members of the 



shelled group subsist on algae and animalcules, and themselves yield the chief 



food-supply of their naked relatives. 



„. , Far more numerous, and therefore of more importance to the 



Bivalves. ^. ■ *■ 



student of distribution, are the bivalve molluscs, or relecypoda, among 

 which are included certain modified forms, like the so-called ship-worms, whose 

 shell, although starting with two valves, eventually assumes a tubular form. These 

 ship-worms (Teredinidw) bore only in wood, where their tunnels take all 

 sorts of directions. In the common European Teredo navalis the pair of long 

 siphons, which when at rest are included in the tube, do not exceed 8 inches in 

 length, whereas in T. arenaria, of the Indian Ocean, they may grow to a couple of 

 feet. In the piddocks or pholases, on the other hand, as typified by the common 

 vol. 11. — 20 



