266 



METAZOAN PHYLA 



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. 365), 

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

 From preserved specimens. A, upper surface. X 3^. B, under surface of female to 

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

 attached to the swimmerets. X M- C, under surface of body of male to show the 

 narrowness of abdominal metameres. X M- 



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 11 

 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 

 known as sow bugs (Fig. 170) and pill bugs, live under stones, boards 

 and other objects upon the ground and are also found in damp cellars 

 and in greenhouses, where the air is moist. 



