The skeleton shrimp, Caprelta aequilihra, is an elon- 

 gated amphipod about 1 inch long. Caprellids crawl 

 about on seaweed, as shown here, or on eelgrass or 

 hydroids. (England. D. P. Wilson) 



body plan as North Atlantic lobsters, but they live in 

 fresh waters. Curiously enough, they are almost en- 

 tirely absent from the tropics, which divide the asta- 

 cids (such as Astaciis and Cambarus) of Eurasia and 

 North America from the parastacids of South Amer- 

 ica, New Zealand, Tasmania, Australia, New Guinea, 

 and Madagascar. Africa itself has no crayfishes at all. 



The largest crayfishes are the Tasmanian Asta- 

 copsis franklinii of surprisingly small streams. These 

 creatures reach nine pounds in weight. American 

 crayfishes are considerably smaller. Yet in the lower 

 Mississippi Valley they are hunted at night for food, 

 either for the tail meat itself or to give flavor and 

 protein to the thick soup known as crawfish bisque. 

 In the same regions, crayfishes often are a pest to rice 

 farmers, for they graze at night on the rice and then 

 retire to the concealment of shallowly subterranean 

 burrows in which water stands for most of the year. 



Lobsters, crayfishes, shrimps, and prawns are of- 

 ten grouped as the long-tailed decapods, the Ma- 

 crura. With them should be included the hermit 



crabs, which adopt empty snail shells as covering for 

 their unarmored abdomens. Hermits (Plates 101, 

 104) are very common on every coast. They become 

 familiar to beachcombers because they run about in 

 daylight where the water is quite shallow, or come 

 out on land in search of food. 



The appendages which in a lobster form the sides 

 of the water-scooping tail fan have become modified 

 in hermit crabs and serve in holding to the inside of 

 the snail shell. When disturbed a hermit draws back 

 quickly into its shelter, often leaving only the tips of 

 its two big pincer claws exposed, simulating protec- 

 tive doors. When left to their own devices, however, 

 hermit crabs investigate every empty snail shell they 

 encounter, often trying out an alternate for size. In 

 this way they trade shells at frequent intervals and 

 keep up with their own growth. To human collectors 

 of shells, this habit can be most aggravating. A col- 

 lector who leaves a fine specimen beside an ant nest 

 for the ants to finish cleaning may find that a hermit 

 crab has come along, carried off the collector's shell, 

 and left an old, dirty, worn one of a common kind. 



Largest of the hermit crabs is Birgiis latro, the 

 robber crab of South Pacific islands. After the cus- 

 tomary juvenile period in the sea, the robber crab 

 becomes progressively more terrestrial. Young ones 

 may use a large snail shell or a small coconut as a 

 cover for the abdomen. But gradually a robber crab's 

 armor becomes heavier and the crab ceases to burden 

 itself with shells. At the same time its abdomen twists 

 toward a symmetrical position, although on its under- 

 surface the crab has abdominal appendages devel- 

 oped on only the left side. 



With its great pincers, the robber crab can open a 

 variety of containers; it has earned its name by feed- 

 ing from human utensils. It can also hammer and 

 pick at a coconut husk until it reaches the contents of 

 the seed within. If necessary, the crab climbs coconut 

 trees and cuts the nuts from their attachments. Is- 

 landers who enjoy Birgits as food are said to take 

 advantage of this habit by winding a thick layer of 

 cloth around coconut palms well above the ground. 

 The crabs will pass the cloth in climbing the palm, 

 but on the downward return to the beach, they re- 

 spond to the cloth as though it were the ground and 

 let go of the trunk. Birgus is heavy enough that a 

 fully-grown one, more than 1 foot in length, is likely 

 to cripple itself by the fall and be unable to run away 

 — letting the islanders capture it. 



TRUE CRABS 



True crabs are the short-tailed decapods. 



the 



Brachyura. Their bodies seem to be entirely cephalo- 

 thorax because the abdomen is held curled under- 

 neath, usually fitting between the bases of the legs. 

 In female crabs the abdomen forms a broad flat 

 apron, whereas in males it is narrower and com- 



238" 



