6 LIFE BY THE SEASHORE. 



animals. Thus some worms, like Serpula and Spirorbis, make 

 white limy tubes and shells into which the whole body can 

 be retracted. The acorn-shells, which are often the common- 

 est of all animals on the shore rocks, are Crustacea which 

 secrete a limy shell into which the whole animal can be 

 withdrawn, and which can then be closed to prevent evapora- 

 tion of moisture. In these cases, however, the animals are 

 completely sedentary, never moving from the place where 

 they have settled down in youth, and from their size and 

 shape offering little opportunity to the waves. It is other- 

 wise with the Molluscs, which frequently possess consider- 

 able power of movement, and have, as it were, to consider 

 both the necessity of protection from drought and from the 

 destructive force of the breakers. We are just beginning 

 to understand the significance of the shapes of shells con- 

 sidered from these points of view, and some of the more 

 obvious adaptations only can be pointed out here. 



Most of the molluscs of the shore have either a shell 

 composed of two valves, like cockles, mussels, and their 

 allies, or have univalved shells like limpets, periwinkles, 

 and whelks (Gasteropods). Among the latter the limpet 

 represents the simplest though perhaps not the most primi- 

 tive condition. Its shell is simply conical, and protects the 

 dorsal region of the animal only ; but as everyone knows 

 the limpet has extraordinary clinging power. The thick 

 shell prevents loss of water by evaporation, the firm attach- 

 ment prevents dislodgment by the force of the waves. The 

 majority of the univalved Molluscs on the shore differ from 

 the limpet in possessing a spirally coiled shell, which is 

 often exceedingly thick and dense, and into which the 

 whole animal can be withdrawn. Such forms as periwinkles, 

 whelks, tops, dog-whelks and others do not cling like the 

 limpet, but when alarmed or attacked often drop suddenly 

 from their attachment. As they do so they withdraw com- 

 pletely into their shells, and close the opening behind them 

 by a shutter, or operculum, which exactly fits the orifice 

 (see Fig. 70, p. 244). This done, the animal is completely 

 encased and protected from extremes of temperature. The 

 shell is so dense that the breakers do relatively little harm, 

 even though the animals are rolled about roughly enough. 

 It is believed that the shape and the sculpture of the shell 



