Handbook of Paleontology 95 



the contrary, rocks with a low angle of dip permit the 

 waves to sweep the whole surface and rocks barren of 

 life are the result. Igneous rocks under the erosive 

 action of the sea often present a smooth, rounded sur- 

 face which does not give a good foothold. Some species 

 of animals are protected by the burrows which they make 

 for themselves, and among these are bivalves of the 

 genera Saxicava, Pholas, Petricola and Teredo. Saxi- 

 cava rugosa burrows in rocks and these burrows in lime- 

 stone rocks are often occupied by one of the polychaete 

 worms (Eulalia viridis). Pholas gouges holes in both 

 rock and wood and sometimes, like Petricola, in hard 

 clay. Teredo navalis, the "Ship Worm," burrows in sub- 

 merged timbers and has done great damage to ships, piles 

 of wharves, buoys etc. There are many forms of these 

 boring mollusks in southern waters. There is another 

 very interesting type of shelter. Elaborate grottos are 

 formed by the tubes of one of the tube-building worms 

 (Sabellaria alveolata) and these offer shelter and protec- 

 tion to whole associations of shore animals. 



Many of the shore forms are attached. Some of them 

 are permanently fixed, as sponges, hydroids, bryozoans, 

 corals, tunicates etc. and these die if they become de- 

 tached ; others, like the sea anemones, are normally fixed 

 but are capable of changing their foothold; then there 

 are groups such as the gastropods and chitons, which are 

 attached to rocks or seaweeds by a broad foot. The area 

 of attachment of sedentary animals is in general well sup- 

 plied with mucous glands through which attachment is 

 effected. This is true of most of the gastropods, though 

 the Limpets probably adhere in the same way as the 

 anemones by a very close apposition of the pedal disc. 

 The Acorn Barnacle has definite cement glands. The 



