THE GENERAL CHARACTERISTICS OP SHORE ANIMALS. 7 



are all of importance in giving strength to the shell, and in 

 minimising the danger of rough usage. How successful as 

 a protection the shell must be is demonstrated not only by 

 the great abundance of periwinkles, whelks, etc., on the 

 rocks, but also by the way in which they expose themselves 

 to view when the tide ebbs, braving the dangers of frost 

 and sun. 



The Bivalve shell seems on the whole less efficient as a 

 means of protection, at least very few Bivalves live on the 

 rocks in the exposed way in which the periwinkles and dog- 

 whelks do. Some, like the mussels, grow in great colonies 

 in sheltered places, very many live buried in sand, not a 

 few burrow in rocks, but most are very liable to wholesale 

 destruction in storms. As a rule the Bivalves have little 

 power of locomotion ; they often spin a mass of silky 

 threads, by means of which they anchor themselves to solid 

 bodies, and which, as in the mussels, may constitute their 

 chief defence against the force of the waves. 



Analogous to the habit of shell-making is the process of 

 tube-building, which is carried on by hosts of worms. In 

 most cases the tube consists of an organic substance secreted 

 by the animal, to which are added foreign particles such as 

 grains of sand, or fragments of stone and shell. Among 

 the tube-building worms are the "sand-mason" (Terebella), 

 a very common form, Sabellaria, a social worm, which builds 

 sandy tubes, and many others. In many cases these tubes 

 must be looked on as chiefly a means of protection against 

 organic foes, but in other cases they are strong enough to 

 protect the animal from the. dangers of its physical environ- 

 ment. 



By far the most effective method of protection against 

 these dangers is, however, the habit of burrowing. A 

 burrowing animal obtains protection from the waves, save 

 in great storms; it obtains permanent moisture, a more or 

 less even temperature, and finally is safe from the persecu- 

 tion of most organic foes. The list of benefits is so long 

 that it is no wonder that so many different kinds of animals 

 have acquired burrowing habits. We can mention only a 

 few of them. If you stoop under overhanging ledges of 

 rocks, or turn over weed-incrusted stones, you may often see 

 numerous holes in the rock, from each of which a red star 



