THE INVERTEBRATE LEGIONS 117 



sand bug filters plankton brought to it by the waves by means of its feathery 

 antennas. The spiny lobster, PamiUnis, has no claws, but can inflict cuts with 

 the spines that cover its bodv and antennas. It makes a rasping noise when it is 

 seized by rubbing a knob at the base of the antenna on a rough groove on the 

 face below the eyes. 



Very few of the shrimps and prawns have claws. They eat plankton and 

 smaller bits of organic matter. Very often, these animals travel in large schools, 

 and most of them swim actively, whereas the crabs and lobsters walk on the 

 bottom and swim very little. Perhaps the most famous shrimp is the snapping 

 shrimp, Crangon, which possesses one huge claw. This claw is used to make a 

 snapping or crackling noise that is well known to divers and which became 

 famous during the last war when it was picked up on underwater sound 

 devices. Schools of snapping shrimp are found in shallow, calm water the 

 world over. The marine shrimp, Stenopus, is a very beautiful, clawed shrimp 

 with a white bodv crossed with blue and red iridescent bands and long, 

 filamentous antennas. 



One of the largest and probably the most famous of all crustaceans is the 

 Atlantic lobster Homarus. It grows to edible size of 1 to 3 pounds in about five 

 years, but lobsters may reach very old age, a weight of up to 35 pounds and 

 2 feet in length. Homarus is probably the finest-tasting lobster found anywhere. 



Some crabs have somewhat reduced the abdomen and must hide this fleshy, 

 poorly protected part in a snail shell which is carried about like a house. These 

 are the hermit crabs, which have some remarkable symbiotic relationships with 

 sponges, hydroids, and anemones. 



The true crabs reduce the abdomen to a little tab that folds under the 

 cephalothorax. They form a large and complex group found the world over. 

 Some true crabs have even taken to terrestrial life in the tropics. Crabs are the 

 scavengers of the sea and will eat any organism they can catch or find dead. 

 Their habits are not as varied as might be supposed. They all have large claws 

 which are used for defense and offense or for manipulating objects. The claws 

 of some are to be a\'oided since they can quite easily crush a finger or a toe. 

 Most crabs display their claws in a threatening gesture whenever they are 

 approached. Some crabs are exceedingly well camouflaged and will not threaten 

 with their claws since they depend on concealment for defense. 



Crabs are pugnacious and active animals. Some of the most amusing experi- 

 ences under water can result from watching their antics, their battles with one 

 another, and their rapid, sidewise scuttlings about for food. One crab that is 

 often seen about coral reefs is the spiderlike arrow crab, Stenorhynchus (/ig. 

 36). This bizarre and gangling little fellow makes a humorous picture as it 

 reaches out for food with its blue claws and brings the food to a mouth that is 

 far under its long snout. 



Arachnids: Class Arachnida — Figure 39 



This class is mainly a terrestrial one and includes the economically beneficial, 

 but not well-loved spiders, centipedes, and millipedes. The rightfully disliked 

 ticks are also arachnids. Arachnids were among the largest in size and most 

 dominant of marine animals in the Cambrian Period from 400 to 500 million 



