Sa THE BIOLOGY OF THE SEA-SHORE 



3. Stratum of cockle (Cardium edule), i to 2 inches below 

 surface. 



4. Stratum of the two bivalves Macoma halthica and 

 Tellina, 2 to 3 inches below surface. 



5. Stratum of lug- worm {Arenicola marina). 



No doubt further subdivisions could be enumerated if 

 the other species of Annelida which are usually present 

 were more carefully collected. We agree with Farran 

 (op. at.) in considering that the variations in the fauna of 

 sandy beaches depend largely on the purity of the sand, 

 and the proportion of mud present. The plants Zostera, 

 Ruppia, etc., modify sand faunas considerably, as many 

 animals live on the blades, and many worms among the 

 roots. The slope of a beach modifies its capacity for retain- 

 ing moisture, and is thus of importance. In fine clean sand 

 Farran found the anemone Cylista undata (on C. edule), 

 4 Nemertea, 3 Echinoderms, 17 Molluscs : Tellina, 

 wedge shell (Donax), Tapes, cockle (Cardium), etc.), and 

 3 species of Crustacea, 24 of Polychaeta, and the Sand Eel, 

 Ammodytes (of. Plate VII). 



Most of the sand-dwelling species of Mollusca are 

 burrowers, and being subjected to considerable pressure 

 from shifting of the surrounding loose material, have in 

 many cases developed stout, more or less globular shells, 

 such as Isocardia, Cardium, Venus, and others. In propor- 

 tion, however, as the bivalves seek shelter from the strains 

 of the shifting sands, either in quieter waters, or by burrow- 

 ing deeper, so the shell in response tends to become less 

 heavy and solid, and to assume a flatter shape, permitting a 

 more rapid passage down into the silt (see Fig. 5, p. 74). This 

 is seen in Tellina, Scrohicularia, and others. This deeper 

 burrowing is accompanied by a lengthening of the siphons 

 to ensure proper respiration, and this in turn results in the 

 prolongation of the posterior portion of the test, to house 

 them, until in the deepest burrowers such as the " Gaper " 

 (Mya) and " razor fish " (Solen), the closed shell is abandoned, 

 and the valves function solely as fenders against lateral 

 pressure from the surrounding silt (Woodward, 1907). 



