CRUSTACEA 



261 



and other objects for safety when the tide goes out. Other larger crabs, 

 including the edible ones, find it less easy to hide and follow the water 

 out with the tide. A soft-shelled crab is one which is caught in the process 

 of molting after it has shed its old shell and before the new one has 

 hardened. Among the various types of crabs is the hermit crab (Fig. 307), 

 which possesses a soft abdomen and lives in the empty shells of snails. 

 Sometimes the sea bottom along the shore will be covered with what at 



Anfennule 



Anfenna 

 Eye 



Abdomen 



Fig. 159. — The blue or edible crab of the Atlantic coast, Callinectes sapidus Rathbun. 

 From preserved specimens. A, upper surface. X J^. B, under surface of female to 

 show breadth of abdominal metameres between which and the thorax the eggs are carried. 



C, under surface of body of male to show the 



attached to the swimmerets. X yo 

 narrowness of abdominal metameres. 



Xl 



first glance appear to be unnaturally active snails, but which on examina- 

 tion prove to be snail shells containing young hermit crabs. Sometimes 

 these snail shells also bear other animals, such as sponges, hydroids, and 

 sea anemones. Some crabs, known as spider crabs, have very long legs, 

 which give them considerable speed in locomotion. A Japanese spider 

 crab, the largest known crustacean, is said to reach a measurement of 20 

 feet from tip to tip of the outstretched legs. 



The isopods are common in the sea and in bodies of fresh water and 

 are in part terrestrial. They are flattened dorsoventrally and lack a 

 carapace. All the legs are similar in structure except the posterior pair 

 and, in the male, the anterior pair. The terrestrial forms, commonly 



