84 GUIDE TO LOCALITIES. 



would be absolutely destructive to all but the most protected 

 of the littoral animals ; nevertheless one cannot cease to wonder 

 that animals apparently so frail as the hydroids and bryozoans can 

 withstand the continued beating of the surf, even with the protec- 

 tion afforded by the rockweed. This applies especially to such 

 unprotected hydroids as Clava leptostyla, which may not infre- 

 quently be found on the rockweed in the most exposed places. 

 The reason for the luxuriant development of animal life between 

 tide marks on exposed shores is no doubt, as often has been 

 pointed out, the large supply of food furnished in a ground-up 

 condition by the waves. 



It will be interesting to note that most of the common littoral 

 animals of these exposed shores are in some way or other pro- 

 tected from the full force of the waves by their thick shells or 

 other covering. Those same coverings serve them also as pro- 

 tection against the drying effect of sun and air during their 

 periodic exposure between tides. Among hydroids, Sertularia 

 pumila, always found in abundance on the rockweed, is best pro- 

 tected from the force of the waves and the heat of the sun by its 

 operculated hydrothecae. In consequence of the closure of the 

 hydrotheca by the operculum upon the contraction of the polyp, 

 the hydroid can be exposed to a hot July sun for five or six 

 hours, and after that will revive on being placed in water ; while 

 the same exposure will dry the rockweed and render it brittle. 

 The other common hydroid of the exposed shores, Campanularia 

 flexuosa, has its polyps protected likewise by hydrothecae ; but not 

 having the additional protection of the opercula, it is less able to 

 withstand the force of the waves or the drying effect of exposure 

 to the sunlight. In consequence, the usual position of this hydroid 

 is under the rockweed, upon its stems, or on the ledges. Thus it 

 is not only protected from the force of the waves but kept moist 

 by the rockweed, which at low tide covers it like a moist curtain. 

 The common bryozoan of these exposed shores is Alq/onidium 

 hispidum; the soft, thick and furry polyparium of which is suffi- 

 cient protection against the force of the waves and the drying 

 power of the sunlight, especially as this species grows chiefly 

 around the stems of the rockweed, where it is in no danger of be- 

 ing beaten against the rock. 



The gastropods of the exposed rocky shores are protected by 

 thick shells. This is shown by the commonest species inhabit- 



