WATE-CtrEfeElirTS ON SHAIiLOW-SEA FAUNAS. 267 



would apparently be too great an impediment in burrowing in 

 the bard sand, whereas the spineless shell of C. tuberculatum would 

 afford insufficient hold in the soft sand. A very slight advantage 

 from form or sculpture might be of great importance in saving the 

 cockles from local extinction j for in the case of Torbay the struggle 

 for existence is so severe that both species are occasionally washed 

 on shore in sufficient quantities to be carted away for manure. 



Among the Veneridse the following may be cited as provided 

 with special mooring-apparatus, viz. Venus casina and V. verru- 

 cosa, with deep concentric ribs ; and Cytherea dione, with concentric 

 ribs and long spines in addition. 



The two bivalves, Psanvmolia tellinella and Cardivm aculeatum, 

 are fair examples of active and passive resistance to the inroads 

 of wave-currents by animals living close to the surface of the 

 deposit which forms their habitat. Passive resistance in the case 

 of the spinous cockles is facilitated by their ribs and spines, while 

 it is supplemented by the powerful foot, which moors the animal 

 to the sand and enables it to burrow in it, though at a compara- 

 tively low speed. Certain mollusks with smooth cells and slug- 

 gish habits, such as Mya truncata for instance, avoid the wave- 

 currents by living at a safer distance below the surface of the 

 bottom, being enabled to do this by means of the long tubes with 

 which they are furnished. There can be no doubt that the long 

 tubes possessed by many species of burrowing mollusks are 

 valuable auxiliaries in self-defence, more especially in the case of 

 the young, who, from their small size, w ould be affected by much 

 slighter wave-currents than the adults, and would be in constant 

 danger if obliged to live at the surface, or close to the surface, of 

 the deposit that gave them shelter. 



{d) Animals that live on the 8ea-hottom on Sand or Mud. — This 

 group of animals is more interesting, and at the same time more 

 satisfactory to deal with, than either of those hitherto referred to. 

 The special adaptations of structure to resist or evade wave- 

 action exhibit great variety, while at the same time the animals 

 themselves often offer the great advantage to the investigator of 

 lending themselves to the test of experiment. 



The following will serve for a few examples : — The Grastropod 

 Aporrhais pes-pelecani is a sluggish moUusk that frequents 

 exposed areas of sand a few fathoms below the surface of the 

 water. Its long wing-like processes, jutting out on one side of 

 the shell, though affording the animal a broad base on which to 



