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

BOOK 2

limy granules in this situation, and finally in some sea-slugs the shell is absent altogether.

Many gastropods are carnivorous and so the class falls below the moral level of the inageressive lamellibranchia. The whelk, for example, bores through the shells of oysters or periwinkles and sucks out their soft bodies. The majority, however, feed on plants. ‘The garden snails and slugs, rasping away the leaves of plants by means of their tongues, which are set with fine, serried teeth like a file, are a serious nuisance to man, a nuisance that more than counterbalances the edibility of the whelks and limpets and winkles and Roman snails, and labels the class as a whole as a troublesome one.

Some of the marine gastropods are pelagic —instead of crawling slowly over the seabottom they swim freely in the surface waters ; the “foot” is expanded into transparent undulating wings by means of which the animal glides through the water. The ordinary pond-snail can often be seen in a sort of slow, writhing swimming just below the surface of the water. These pelagic gastropods develop this idea.

The third class of mollusca, the Cephalopoda, is more exciting, including as it does the cuttle-fish and octopuses. There are com-

Fig. 86.

THE SGIENCE- OF LIFE

CHAPTER 3

paratively few species, all marine ; nevertheless, its members are the most highly organized molluscs, and perhaps the most highly organized invertebrates. Further, they include by far the biggest invertebrate animals, for giant cuttle-fish have been recorded over fifty feet long. Their name means “ headfooted’; for the “ foot,’ a tongue-like body in the lamellibranch and flat underneath in the gastropod, here grows forward to surround the mouth and is prolonged into flexible arms.

The majority of living cephalopods capture their prey—which consists chiefly of crustaceans and fish—by stalking and pouncing upon it. The circle of writhing arms surrounding the mouth is provided with powerful suckers, by means of which they catch and hold their victims. Once captured, the prey is bitten by a pair of jaws, sharp and curved like an eagle’s beak, and a digestive juice is pumped out into its body from the captor’s mouth. The juice paralyses the victim very quickly, and then it dissolves and partly digests the flesh, the resulting soup being sucked back into the captor’s mouth held close to the captured prey.

The octopuses live mainly on the bottom, lurking in caves and crannies, often building

LR Bacar.

The Common Whelk (Buccinum undatum) coming out of its shell and attacking a clam

with its long proboscis.

The speckled, uplifted tube is the “ siphon”? through which the whelk breathes water. Below on the right is a cluster of tough, papery whelk’s eggs.

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