200 MARINE ANIMALS 



wood-boring shipworm, Teredo (Pholadidae) ; and Lithodomus (My- 

 tilidae). Sea urchins bore even into lava and metamorphic rock when 

 exposed to wave action, 25 thus Echinus militaris, Strongylocentrotus 

 lividus (which does not bore in the Mediterranean!), and species of 

 Arbacia, Cidaris, and Heterocentrotus. These borings are often so 

 numerous as to be separated only by thin walls; the external openings 

 of the tubes are frequently narrower than the tube itself, the size of 

 which is adjusted to the growth of the animal, so that they are con- 

 fined to this location for life. Such intertidal borings are found only 

 so placed that they retain water at ebb tide, and the sea urchins are 

 enabled to live between tide marks, where at the surface they would 

 be unable to withstand drying. Among barnacles, Lithotrya bores in 

 rock, and Alcippe into small shells. 



The holes provided by these rock-borers, like all other crevices, are 

 made use of by other forms. The ark shell, Area barbata, the peculiar 

 echiurid worm Bonellia, and the small claw-snapping decapod Alpheus 

 are especially common in the borings of Pholas. 



The animals living unconcealed on rock surfaces are often provided 

 with means of defense. The spicules of sponges and the stinging cells 

 of the coelenterates have already been mentioned. Snail shells are thick 

 walled, tough, and often beset with spines. Shells of lamellibranchs are 

 thick, arched, strengthened by corrugations, and often covered with 

 folds or spines. Crustaceans have a thick, spiny shell. The name 

 "echinoderm" indicates the nature of protection in this group, at least 

 in the sea urchins and starfishes. These shell characters may be a re- 

 sponse to the physical rather than to the biotic environment. 



Other rock inhabitants obtain protection through protective re- 

 semblance; the Octopus sits in openings among the stones whose color 

 is imitated by its skin, and the power of color change protects it when 

 it changes location. Many fishes are able to adapt their coloration to 

 that of the bottom. The scorpaenids, lying in wait for their prey, press 

 themselves close to their support, and with their dermal appendages 

 of varied form and their wonderful color resemblance, appear like 

 pieces of alga-covered rock (Fig. 34). Many crustaceans, Hyas, 

 Inachus, and Maia, for example, which maintain themselves among 

 rocks covered with other animals, have the extraordinary habit of 

 attaching algae, sponges, polyps, and tunicates to their carapaces. 26 

 If one removes this covering, they promptly replace it, as they must 

 also do when they cast their skins. Each consequently adapts itself to 

 its special surroundings ; thus where the sea lettuce Ulva predominates, 

 they will be clothed with it, while if a varied algal growth is present, 

 their carapaces will reflect its variety. Crabs of the genus Dromia bear 



