50 THE BIOLOGY OF THE SEA-SHORE 



and bulbosa. The roots are exceptionally exposed to the 

 air and the following, among others, inhabit them : 15 species 

 of Turbellaria, 12 of Nemertea, 67 Polychasta, Phascolosoma, 

 6 Nudibranchs, and many others. Farran suggests that 

 collecting from Laminaria roots would seem to furnish 

 a reliable basis for the comparison of littoral faunas of 

 different countries. As many as 66 species of worms have 

 been obtained from half a dozen roots. The method of pro- 

 cedure is to tear the plants off the rocks, cut away the stems, 

 and submerge the roots in water, in dishes. Some species 

 soon collect around the margin, others require prolonged 

 soaking, and a succession emerge in two or three days. At the 

 end of three days putrefaction begins, and tube-dwellers usually 

 emerge and fall to the bottom of the dish. Now add a few 

 drops of formalin to hasten the process, throw away the roots, 

 and examine the debris at the bottom of the dish. 



Zoning of the Animal Forms of a Rocky Shore. — 

 Zoning is very well exhibited by the Trochidae and Lit- 

 torinidae of the shore. Investigations in Cardigan Bay 

 revealed the presence of nine species which (allowing for 

 local differentiation and overlap) show the following succes- 

 sion, from above downward : 



Littorina neritoides. 



L. rudis. 



Monodonta crassa. 



L. obtusata, 



L. littorea. 



Gihhula umhilicata, G. magus. 



G. cineraria. 



Calliostoma zizyphinus. 



The presence of L. obtusata is dependent on that of Fucoid 

 Algae ; C. zizyphinus is rare in that area and G. magus, local ; 

 L. neritoides is constant when conditions are suitable, and 

 requires a rocky foreshore affording crevices just above high- 

 water mark, and not exposed to the midday sun and the 

 resulting desiccation (Walton, 19 13). In other countries 

 different species of periwinkle inhabit the supra-littoral zone ; 



