The science of life : fully illustrated in tone and line and including many diagrams

FURTHER PATTERNS OF INDIVIDUALIZED ANIMAL LIFE

tary muscles of a vertebrate or an arthropod ; their locomotor apparatus consists generally of an apparently disorderly tangled mass of muscle-fibres, the so-called “ foot,’ an arrangement which leads to plasticity rather than precision.

There are five classes of molluscs, of which only three, the bivalves, the snails and slugs, and the octopuses and cuttle-fish are of sufficient importance to mention here.

The bivalves, or Lamellibranchia, are all aquatic and most of them marine. They lie between two hard shells which are hinged together along one side ; by means of powerful muscles the shells can be pulled together to enclose the animal completely when any danger threatens. The poor thing’s only retort to hostility in most cases is just to “slam the door.” Ifman must go to the ant, that arthropod, for lessons against sluggishness, it is to the lamellibranch he must be sent to calm his violence and anger. Some of these animals, like the fresh-water mussel, creep sluggishly about by means of that lobe of muscular tissue, the “ foot,” which they protrude from the shell. Some, like the heavyshelled oyster, or the common blue mussel of our shores, that attaches itself firmly to rocks by means of a tuft of adhesive threads, do not move about at all. Some, like the cockle, which can jump by means of a spring-like action of its ‘foot,’ or the scallop, that swims by rhythmically opening and closing its shell-flaps, are surprisingly active creatures. ‘The scallop is a marine butterfly ; to guide it in the long flights which it takes it is provided with a row of glistening eyes just inside the rim of each shell. To ourselves the bivalves are important for three chief reasons ; firstly, because most of them are good to eat, secondly, because the oyster makes pearls, and, thirdly, because some members of the class bore holes in wood or rock, and can do extensive damage in this way to piles, breakwaters, wooden ships, and the like.

The bivalves live by passing a continual current of water through the shell, a current from which they extract oxygen and the minute particles of organic matter upon which they feed and into which they shed their waste-products. On opening a mussel, the most obvious structures that present themselves are the so-called “ gills,” the engines which keep this slow current of water in motion, The “ gills? are not primarily

closing shell

I: SNS ey Es Muscle for ii Xe i \

fig. 85. Showing

breathing organs at all ; they are elaborate filters, like the pharynx of Amphioxus or a sea-squirt, through which water is worked by means of cilia, and which collect suspended particles of food and pass them down to the mouth.

Some of the most important points in the anatomy of these creatures may be seen in the figure below. Perhaps their most bizarre feature, in vivid contrast to the scheme of the two phyla previously studied, is the fact that the intestine runs actually through the ventricle of the heart.

The second class of molluscs, the Gastropoda, is also the largest; it includes the terrestrial snails and slugs and the whelks and winkles of the sea. There are about 6,000 terrestrial and 10,000 aquatic gastropod

Stomach

Liver

Heart

Intestine

Kidney

Oe /;

a Varig ign N Eyes Gill A Scallop after removal of one valve of the shell, where some of the more important organs are placed.

species living to-day, most of the latter being marine. With them the “foot” is a flat muscular sole to the body lump—hence the name of the class, from the Greek for stomach and foot. ‘They go on their stomachs ; and most of them bear on their back a spirally coiled shell into which they can retire when they choose. In some forms the tail of the “foot”? carries a hard chitinous plate, so arranged that when the animal retires into its shell the plate closes the opening of the latter like a lid ; in some, such as the powerfully adhesive limpet, the shell is not spirally twisted at all, but forms a flat cone under which the animal cowers ; in some, such as the sea-hare, it is of no use as a protection, being represented by a mere thin plate of chitin embedded in the flesh of the back ; in many garden slugs there are only a few

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