270 



THE MOLLUSKS 



Scallop. — The scallop, another molluscaii delicacy, forms an impor- 

 tant fishery. Only the single adductor muscle is eaten, whereas in the 

 clam the soft parts of the body are used as food. 



Pearls and Pearl Formation. — Pearls are prized the world over. It 

 is a well-known fact that even in this country pearls of some value are 

 sometimes found within the shells of such common bivalves as the fresh- 

 water mussel and the oyster. Most of the finest, however, come from 

 the waters around Ceylon. If a pearl is out open and examined carefuUy, 

 it is found to be a deposit of the mother-of-pearl layer of the shell around 

 some central structure. It has been believed that any foreign substance, 

 as a grain of sand, might irritate the mantle at a given point, thus stimu- 

 lating it to secrete around the substance. It now seems hkely that most 

 perfect pearls are due to the growth within the mantle of the clam or 

 oyster of certain parasites, stages in the development of a flukeworm. The 

 irritation thus set up in the tissue causes mother-of-pearl to be deposited 

 around the source of irritation, with the subsequent formation of a pearl. 



Gastropods. — Snails, whelks, slugs, and the 

 Hke are called gastropods, because the foot 

 occupies so much space that most of the organs 

 of the body, including the 

 stomach, are covered by 

 it. Such animals are par- 

 tially covered by a more 

 or less spirally formed 

 shell which has but one 

 valve. In most gastro- 

 pods the body is spirally 

 twisted in the shell. In 

 the garden slug, the man- 

 tle does not secrete an external shell, and the naked 

 body is symmetrical. 



Gastropods of various species do considerable 

 damage, some in the garden, where they feed upon 

 young plants, others in the sea, where they bore into 

 the shells of other living moUusks in order to get 

 out the soft part of the body which they use as food. 

 Cephalopods. — Another class of mollusks are 

 those known as cephalopods. The name "ceph- 

 alopod" means head-footed. As the Figure shows, 

 the mouth is surrounded with a circle of tentacles. 

 The shell is internal or lacking, the so-called pen of 

 the cuttlefish being all that remains of the shell in 

 that form. A cuttlefish is strangely modified for the 

 life it leads. It moves rapidly through the water by squirting water from 

 the siphon. It can seize its prey with the suckers on the long tentacles 



Forest snail, showing the two 

 tentacles with an eye on 

 the end of each. From 

 photograph by Davison. 



The squid. One fourth 

 natural size. 



