168 DISTRIBUTION. 



rock-pools, but in that portion of the belt which is left uncovered during the ebb, and where they 

 meet with conditions entirely different from those w hich are present in the deeper zones from 

 which the sea never retires. 



Among the hydroids which contribute more especially to characterise the Literal Zone may 

 be mentioned the following : 



Clava squamafa, Coryne puHiIla, Clavatella proliferci, Laoiiicdea Jlexuosa, GonoUiyran Loveiii, 

 Sertularia pumila. 



Of these, Clavatella prol if era is confined to the rock pools, occurring In the smaller pools 

 which are situated near the level of high-water neap tides ; while the others attach themselves to 

 the moist rocks or to the surface of the seaweeds which are left exposed during the ebb. 



III. The Laminarian Zone. — This zone embraces the average range of that portion of the 

 shore which is uncovered only at spring tides. It is the favorite abode of numerous hydroids, 

 and since the sea ebbs from it as in the Litoral Zone, it presents both rock pools which at all 

 times retain a supply of water, and stretches of shore from which the sea entirely retreats. 



Unlike the Litoral Zone, however, it is only during spring tides that this zone becomes 

 exposed. Instead, thei'efore, of l)eing laid bare to the atmosphere twice in every twenty-four 

 hours, there are two intervals of nearly a fortnight in each month, during which the sea never 

 leaves it, and even then — since it forms only a narrow baud at the extreme limit of low water — 

 it is but for a short time at each ebb that it continues exposed. Its conditions are thus different 

 from those of the Litoral Zone, and as may be expected, it possesses a different fauna. 



The fauna of the Laminarian Zone is a very rich one. This region is the abode of such species 

 as cannot endure the long and repeated exposure to the atmosphere to which those of the Litoral 

 Zone are subject, and which yet suffer no injury from such sjiort withdrawal of the water as they 

 experience after each of the long intervals which intervene between every access of spring tides. 



The species which frequent the rock pools are here, as in the case of those with a similar 

 habitat in the Litoral Zone, less decidedly limited to one bathymetrical area than are those which 

 attach themselves to exposed rocks and seaweeds. The Laminaria digitata, associated with 

 Rhodomenias and other red and purple Algae, botanically characterises this zone round the shores 

 of the British Islands, and of the other more northern parts of Europe, and some of the most 

 distinctive hydroids of the region may be seen congregating round the roots and stems, or 

 spreading over the broad fronds of this grand seaweed. In the Mediterranean, with its almost 

 tideless shores, the place of the Laminaria is taken chiefly by beautiful Cysioscirce, whose 

 narrow heath-like ramifications give support and shelter to iiydroids of other species than those 

 of our northern laminarian fauna. 



The following species may be cited as among the most characteristic hydroids of the 

 Laminarian Zone of Britain : 



Tuhularia larynx, Tiibularia indivisa, Tuhularia bellis, Coryne vayinaia, Syncoryne eximia, 

 Myriotliela arciica, Ohelia geniculata, Camjxinularia Jolinstoni, Sertularia operculata, Sertularella 

 rugosa, Aglaoplienia pduma. 



IV. The Coralline Zone. — This zone extends from the lowest level of spring tides to a 

 depth of about fifty fathoms from the surface, and is never exposed in any state of the tide. Its 

 bottom varies much, and affords appropriate habitats for correspondingly varied forms of life. 



