220 THE LIFE OF CRUSTACEA 



power of locomotion, and, as females bearing eggs 

 are never taken on fish, it would appear that they 

 drop off after gorging themselves with blood, and 

 probably seek a retreat at the bottom of the sea, 

 where they may hatch their young in safety. The 

 digestive canal of ^ga dilates into a large bag, 

 which becomes distended with a semi-solid mass of 

 blood. This mass, when extracted and dried, is the 

 ** Peter's stone " of old Icelandic folklore, to which 

 magical and medicinal virtues were attributed. The 

 species j^ga spongiophila, already mentioned, differs 

 in its habits from all the other species of the genus, 

 since it lives, not on fish, but in the interior of a 

 sponge. 



The most completely parasitic members of the 

 Cymothoidae are found in the subfamily Cymo- 

 thoinge, including the typical genus Cymothoa 

 (Plate XXIX.) and many others. The adult animals 

 are found clinging to the skin of fishes, the legs 

 being provided with strong hook-like claws that give 

 them a very firm hold. Some species, especially 

 common on Flying-fishes, cling to the tongue of the 

 fish, and almost prevent it from closing its mouth. 

 When young, the Cymothoinae swim freely, and the 

 shape of the body is not unlike that of the Ciro- 

 laninse ; but after they have settled on a host the 

 body often becomes distorted and unsymmetrical. 

 A still more remarkable change occurs in the repro- 

 ductive organs in some, if not in all members of this 



