IC2 



Invertebrata. 



a dorsal shield armed with a strong median spine 

 (fig. 57, A) and followed by a jointed post-abdomen 

 which bears no appended limbs. 



A closely allied animal common on our shores is 

 the hermit crab, which protects its soft, almost limb- 

 less post-abdomen by inserting it into the deserted 

 shell of a whelk, or other univalve mollusc. In these 



FIG. 57. 



Stages in the development of the common Shore Crab (Carcinus matnis). 

 A, First or zoea stage ; B, Megal 

 stages of growth. All magnified. 



. 

 A, First or zoea stage ; B, Megalopa stage with tail ; c, D, advanced 



ll 



the pincers are usually unequal, so that on the 

 animal being molested, one can be retracted while 

 the larger one blocks up the passage. The soft 

 abdomen acts as a sucker, whereby the hermit crab 

 retains its hold on its habitation. 



There are many varieties of form among Crustacea, 

 and those above described are among the most highly 

 organised, all having stalked eyes and ten walking 

 feet. The mantis shrimps have their thoracic limbs 

 fitted for walking, as well as the abdominal legs, so 

 that instead of ten, there are fourteen or sixteen legs. 



