250 PROF. \V. A. HEKUMAK ON THE ABUNDANCE 



generally from the erosive action of the sea. On Hilbre Island, in the 

 Estuary of the Dee, for example, Sabellaria covers the soft red triassic 

 sandstones with encrusting sheets, rising in places to form massive hummocks 

 and outstanding reefs many yards in extent ; and the same reef-like 

 formations are to be seen on a still larger scale on the coast of Normandy, 

 near Granville, Mount St. Michel Bay, and elsewhere. 



I have measured and counted numbers of samples of these Sabellaria 

 tubes, and find that the diameter of the mouth of the tube is generally 

 between an eighth and a tenth of an inch. Of course, there are also smaller 

 ones, those of the young worms in the mass, and a few larger, but a fair 

 average size is one-tenth of an inch. A photograph (fig. 1) of such a mass 

 of Sabellaria tubes shows that in a surface of about three square inches there 

 are from 05 to 75 mouths of tubes. Taking 6!>, a square foot would have 

 3120 and a square yard some 28,080 of the worms. Hence, forty square 

 yards would contain over a million. Now, there are very many square yards 

 of Sabellaria on the shore at such a locality as Hilbre Island, containing, 

 therefore, millions of worms, each from one to two inches in length, and such 

 Polychset worms are a favourite food of flat-fish like the plaice and sole, and 

 can be easily obtained by crunching up the brittle sand-tubes. Boughly, 

 about half of a mass of Sabellaria tubes consists of sand and the rest is 

 formed of the nutritious worms. 



Sabellaria is not found on the shore only, but extends beyond low-water 

 mark for some distance. I have dredged lumps of it from depths of about 

 10 fathoms, off the Lancashire coast, near Fleetwood, and in some places it 

 it so abundant at the bottom, and comes up so frequently in the fishermen's 

 nets, that it is familiar to them, and is known by the local name of " knarrs." 

 That flat-fish are present on this trawling ground is probably due in part to 

 the abundance of the Sabell&ma masses on the bottom. 



Balanus. 



The common rock-barnacle Balanus balanoides is probably the most 

 abundant fixed animal on the rocky shores of North-West Europe. On 

 many sea-cliffs the barnacles form a layer covering practically every inch of 

 rock between the tide-marks. Bradda Head at Port Erin is a good example, 

 and at low tide, when seen from a distance, it looks as if a horizontal band 

 of whitewash had been painted along the base of the cliff. When examined 

 in detail, it is found that the barnacles — young and old — form a continuous 

 layer, roughening the whole surface of the rock and leaving scarcely an}' 

 spots of stone exposed. In fact, the individual barnacles in many cases have 

 not room to grow to the normal size. They are overcrowded, and by mutual 

 pressure are caused to assume vertically elongated prismatic forms, like 

 columnar basalt. A quarter of an inch square is an average size for an adult 



