660 THE BIOLOGY OF MARINE ANIMALS 



rock. During this process the animal fastens itself by its foot, and the 

 valves rock to and fro, scraping away the walls. 



Other common forms of bivalves with similar habits are Bamea, 

 Pholadidea and Gastrochaend. Hiatella (Saxicava) is a related genus boring 

 into limestone and sandstone. Boring is accomplished mainly, if not 

 exclusively, by mechanical means. During this process the siphons are 

 closed and partly withdrawn, the contained water is pressed into the 

 mantle cavity and the shell valves are forced apart and scraped against the 

 walls of the burrow. Repetition of this process slowly wears away the 

 stone. 



Besides the piddocks, rock-boring habits are found in two other bi- 

 valves, namely Petricola (Petricolidae) and the date mussel Lithophaga 

 (Mytilidae). The shell of Petricola superficially resembles that of pholads 

 and it burrows into the same kind of rocks. Lithophaga, on the other hand, 

 has a thin and fragile shell, devoid of teeth and spines and covered ex- 

 ternally with a thick periostracum (Fig. 15.14). The date mussel restricts 

 its attacks to limestone and other calcareous rocks, which it excavates 

 by chemical means. Special glands in the mantle produce an acid secretion 

 which attacks the stone, while the calcareous shell of the mollusc is pro- 

 tected by the thick periostracum (137). 



Sea-urchins which bore into rock occur commonly in the littoral region 

 in many parts of the world. The excavations vary from shallow depressions 

 in the rock surface to deep cells. Boring is accomplished by mechanical 

 action of the teeth and peripheral spines, and is a means of protection 

 against strong wave action. Paracentrotus lividus, found in the Mediter- 

 ranean and on west European coasts, excavates deep cells on surf-swept 

 shores, but does not display this habit in quiet Mediterranean waters. 

 Paracentrotus is reported to bore into wood as well as stone. 



Wood-borers 



Wooden structures are attacked by two groups of marine animals, 

 namely lamellibranchs and crustaceans. Of these the shipworm Teredo 

 and its allies are the most important. Furthering the destructive activity 

 of the shipworm are various isopods, especially the gribble Limnoria. 



The shipworms (Teredinidae) are highly modified for excavating and 

 living in wood, which provides shelter, protection and food. The body is 

 elongated and almost naked except for a pair of small valves at the anterior 

 end by which boring is accomplished (Fig. 15.14). The valves are globular 

 in shape and are composed of three lobes ; the outer surfaces of the two 

 anterior lobes are provided with fine ridges bearing sharp teeth (Fig. 1 5. 1 5). 

 In shipworms, as in pholads, a hinge ligament is absent, and the inner 

 surfaces of the valves possess articulating knobs on which they rock to and 

 fro. The valves are moved by contractions of the adductor muscles. When 

 the strong posterior adductor muscle contracts, the anterior lobes of the 

 shell are drawn apart and rasp the sides of the burrow. During this activity 

 the shipworm is held in position by the sucker-like foot, and a constant 



