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When scientists began more carefully to examine the structure of shellfish, they found that those similarly constructed had shells with certain marked peculiarities. The days of conchology were then doomed; and the study of the mullusc, or malacology, took its place.
Besides those necessary for digesting food, most shellfish have organs equivalent to those of vertebrate animals, such as feet, arms, eyes, head, heart, and tongue. Although bearing the same names, these organs rarely have a similar shape to those of the vertebrates, being necessarily adapted to the different mode of living. The foot of a cockle, shaped like an animal’s tongue, enables it to move slowly from place to place, as well as to burrow in a sandy beach with the comical jerks so well known to observers. The tongues are beautifully designed for their work. The long, narrow tongue of the vegetarian mollusc works like a scythe, and mows down the delicate marine grasses on which the animal feeds. The powerful tongues of those that prefer an animal diet are able to bore through the strongest shells; and woe betide the unfortunate shellfish which, having shown signs of weakness, or disease, is surrounded by its active, carnivorous brethren. The tongue, sometimes longer even than the shell itself, is covered with rows of very hard spikes, or teeth, arranged similarly to the burrs on a file. As these teeth break, or are worn out, they are replaced by others that push themselves forward when wanted. Under a microscope of moderate power, the radula, or tongue, of a shellfish, especially a limpet, is a most interesting sight, and many molluscs can be identified merely by examining the tongue under a microscope. The shape of the teeth, the number, and the arrangement of them will settle the question.
The appetites of molluscs verge on the voracious. Break up a few cockles, or other shellfish, and place them in shallow water on a calm day, and watch the result. If in the vicinity of rocks, and during a rising tide, all the better. First come the wary little shrimps to the feast. Some are creeping cautiously, and some are jumping and racing, as if afraid of not being in time. Then the carnivorous shellfish approach from all directions, foremost amongst them being the different species of Cominella. While they are lumbering along, shells appear to be actually running; but a close inspection shows that these contain active little hermit crabs, whose tender tails, having no hard covering of their own, are snugly stowed in the empty shells of defunct
