INVERTEBRATE BOTTOM FAUNA 475 



Several families of burrowing mussels inhabit the shell-sand, 

 the most important being Veneridse, Tellinidse, Astartidae, 

 Cardiidse, and Solenidee. The most characteristic species 

 are Venus casina, V. fasciata, Timoclea ovata, the species of 

 Tellina and Psainmobia, Nicaitia banksi, Solen ensis and 

 Cardiiivi fasciatuin. The common cockle, Cardhmi 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 Naticidse, and par- 

 ticularly Lunatia 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, Spatangits purpureus, and 

 Echinocyainus pusillus, the last 

 named being the only clypeastrid 

 in northern seas. Except perhaps 

 Spatangus piupuretis, 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. 



// / / / • 1 • U ■ 1- Echinocardiiini fiavescens, O. F. Miill. 



Asti'opeden irregularis, which ■' 



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 riibens, 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 Astropecten swallows whole little worms, 

 mussels, the young of Echinocardium, and other small animals. 



The worms are chiefly those belonging to the genera 

 Glycera and Nepktkys, and the family Ophelidse {Ophelia lima- 

 cina and Travisia forbesi). They live down in the sand, where 

 they make long passages that are kept open by having the 

 walls lined with a film of slime. 



All these animals are variously equipped for living buried 

 in the sand, which naturally forms a splendid protection against 

 their enemies. The burrowing mussels are provided with two 

 more or less elongated movable siphons, the openings of which 



