. The depths of the ocean; a general account of the modern science of oceanography based largely on the scientific researches of the Norwegian steamer Michael Sars in the North Atlantic. Oceanography. INVERTEBRATE BOTTOM FAUNA 475 Several families of burrowing mussels inhabit the shell-sand, the most important being Veneridse, Tellinidae, Astartidae, Cardiidae, and Solenidae. The most characteristic species are Venus casino,, V. fasciata, Timoclea ovata, the species of Tellina and Psammobia, Nicania banksi, Solen ensis and Cardium fasciatum. The common cockle, Cardium edule, on the other hand,
. The depths of the ocean; a general account of the modern science of oceanography based largely on the scientific researches of the Norwegian steamer Michael Sars in the North Atlantic. Oceanography. INVERTEBRATE BOTTOM FAUNA 475 Several families of burrowing mussels inhabit the shell-sand, the most important being Veneridse, Tellinidae, Astartidae, Cardiidae, and Solenidae. The most characteristic species are Venus casino,, V. fasciata, Timoclea ovata, the species of Tellina and Psammobia, Nicania banksi, Solen ensis and Cardium fasciatum. The common cockle, Cardium edule, on the other hand, never occurs here. Solen ensis is generally so deeply embedded that an ordinary dredge brings up merely fragments instead of the whole animal. The small species of Lunatia belonging to the gasteropod family Naticidae, and par- ticularly Ltinatia intermedia, also burrow some distance down, as they feed on little mussels, boring through their thin shells to get at the animals within. Antalis entalis is often common here. Spatangids are represented by Echinocardium fiavescens (see Fig. 338), the commonest of all, Spatangus purpureus, and Echinocya7nus pusillus, the last named being the only clypeastrid in northern seas. Except perhaps Spatangus purpureus, they are not confined to the shell-sand of the skerries, but may be found also in the clay of the sublittoral zone. All of them burrow deeply. Another deep-burrowing form is Fig. 338. Astropeden irregularis, which ^inocardiumjlavescens, O. F. Mull. also lives in the clay bottom of both the skerries and fjords. This creature has conical legs (without suckers) particularly well adapted for digging, though they compel it to procure its food in a different way from Asterias rubens, which preys on large mussels by placing its foot-suckers on their shells and pulling the valves apart till the muscles relax and the shell is opened, whereas Astropeden swallows whole little worms, mussels, the young of Echinocardium, and other small anim
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