gency reversal of the normal direction of swimming, 

 a scallop's behavior suggests that of the octopus itself 

 or of lobsters and crayfish. 



These same reactions and ability to swim are 

 found in the file shells (Lima — Plates 44-46), whose 

 obliquely oval valves commonly bear close-set and 

 overlapping edges across the ridges that radiate from 

 the narrow hinge. In some species these edges sug- 

 gest the teeth of a file. The two valves are of equal 

 size and are mirror images in shape, whereas a scal- 

 lop's upper (left) valve bulges more than the lower 

 (right) valve. From Lima shells, a double fringe of 

 long, slender, pale tentacles extend, often hiding the 

 minute eyes between them at the mantle edge. 



Lima has a diminutive foot from which the animal 

 can extrude fine threads of a plastic material, the 

 byssus. With these threads a file shell builds a crude 

 tubular nest somewhat resembling those created by 

 some of the wolf spiders. Usually the nest of Lima 

 is under the sheltering bulge of a rock, and is open 

 at both ends. This permits the 1-inch clam to create 

 a feeding current through the nest, entering at one 

 doorway and leaving at the other. 



Scallops too, when quite young, can secrete byssus 

 threads. Most of them give up the habit. Pecten 

 latiaiiritiis of America's Pacific coast is an exception, 

 a small scallop that ties itself to kelp and other sea- 

 weeds by a single byssus strand. Sometimes it uses 

 the byssus gland at the tip of its mobile tongue-shaped 

 foot to hold temporarily to hard surfaces. By this 

 means it can hitch its shell along, even up the verti- 

 cal side of a glass aquarium. 



TETHERED MUSSELS 



Marine mussels {Mytilus — Plate 40 — and Modi- 

 olus) are more expert at using byssus threads. They 

 not only creep along by secreting one short thread 

 after another from the extended foot, but can also 

 hold fast to several dozen strands fanning out like 

 guy ropes at a point of more permanent attachment. 



Mussels the world over anchor themselves in this 

 way to rocks and to each other, sometimes producing 

 a "scalp" several inches thick over a sand bar and 

 retarding erosion by waves. In France and other 

 parts of Europe where Mytilus edulis is appreciated 

 as food, the deep blue iridescent shellfish are culti- 

 vated in shallow coastal waters by providing them 

 with tree branches driven into the bottom, or with 

 other surfaces upon which they can attach themselves 

 and feed. Cultivated mussels usually grow larger 

 than their unaided brethren, and shells over 2 

 inches long are regarded as salable. The horse mussel 

 Modiolus is a larger animal, often living partly buried 

 in mud flats; its flavor is less attractive. 



Byssus threads anchor far larger and more fragile 

 shells in moderately deep and warm water. Pen 

 shells (Pinna) are triangular or wedge-shaped, as 



Garden snails, such as the Helix aspersa introduced 

 into North America from Europe, glide out of damp 

 corners at night and browse on a wide variety of 

 plants, leaving glistening trails of mucus wherever 

 they go. (California. H. S. Barsam) 



much as a foot in length. In some species the byssus 

 is coarse and black, but in Pinna nobilis of the 

 Mediterranean it is silky, fine, and of a bright golden 

 color. From this material in ancient times artisans 

 wove "cloth of gold" (tarentine fabric), particularly 

 in the Greek city of Tarentum (now called Tarento) 

 under the heel of the Italian boot. Museums now dis- 

 play gold-cloth gloves made of this "marine silk," 

 and point out how permanently flexible the material 

 remains. So fine is the texture of this cloth that a pair 

 of lady's gloves made from it can be crushed without 

 harm into the space of a walnut shell. Pinna shells 

 are a source also of black pearls. 



OYSTERS AND OTHER FIXED CLAMS 



One of the scallops, Hinnites giganteus, from the 

 Pacific coast of America, shows how easy is the step 

 from a free-swimming life to a permanently attached 

 one. As a youngster it swims about, flapping its equal 

 shells as though it were a Lima with a smoother shell 

 and shorter tentacles. Then it slips into a crevice be- 

 tween rocks on the bottom and settles down. To the 



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