64 NATURAL HISTORY OF ARTHROPODS. 



number varies in different works from six to sixteen. It is not necessary here to 

 discuss all of the families of the group, but merely to mention some of the more im- 

 portant forms. It is to the Ocypodoidea that the larger number of the terrestrial 

 Decapods belong. An account of their respiratory apparatus has been given on a 

 preceding page. 



The PINXOTHERID^E are familiar to all in the little " Oyster Crab," Pinnotheres 

 ostreum, so common in the oysters of our coast. Many tales are told of the way in 

 which these forms act as guardians of the molluscs inside whose shell they dwell, and 



which have been perpetuated in the generic name, but which 

 an iconoclastic science has shown to be false, these animals 

 seeking this home merely for protection and for convenience 

 of obtaining choice morsels of food in the currents of water 

 created by the cilia on the gills and mantle of the mollusc. 

 With our common oyster crab it is the female alone who thus 

 keeps house, the male living a life of freedom, being very 

 rarely seen. Other forms belonging to this or allied genera 

 have similar commensal habits, some living in the shells of mussels or other bivalves, 

 while one form from the Philippines is said by Dr. Semper to enter in the young state 

 the water-lungs of the Holothurians and there spend its life, the carapax growing for- 

 ward over the eyes, causing blindness or partial blindness, while the eyes themselves seem 

 to undergo a more or less extensive retrogressive metamorphosis. A somewhat similar 

 case is found in the Chilian Pinnaxodes chilensis, which enters the anal opening of a sea- 

 urchin, and sometimes occupies a third of the body-cavity of the host. It is one of the 

 peculiarities of geographical distribution that the common Pinnotheres pisum of the Eng- 

 lish shores also occurs in antipodal New Zealand, occupying in both places what appears 

 to be the same species of mussel. Pinnixa is an aberrant form, some species of which live 

 in the tubes of certain marine worms. In these the body is very short and broad, the 

 breadth being occasionally three times the length. The two genera, Jfarpalocarcinus 

 and Cryptochirus, belonging near the Pinnotheridae, agree in having a sort of pouch 

 for holding the eggs and young, formed by the lateral extension of the abdominal 

 plates, and in living in coral where they produce structures recalling the galls produced 

 upon plants by certain insects. Harpalocarcinus affects the branching forms, and, 

 settling down between two twigs, stays there until by the growth of the coral he is 

 rendered a prisoner for life, two small holes only remaining, through one of which he 

 draws the water necessary for his existence, the other serving as an exit for the water 

 which has served his purpose. The currents of water serve to keep the cavity open 

 as long as the crab lives. Cryptochirus prefers to make his home in the more solid 

 corals, where the young, settling down in the centre of a young polyp, kills it, while the 

 surrounding polyps continuing to grow soon build a tubular dwelling for the crab. 



Passing by the Grapsidaa, witli its several hundred species, we come to a more in- 

 teresting family, the OCYPODID^E. Here belongs the genus Gelasimus. In the males 

 of this genus one of the claws is greatly larger than the other. When these crabs are 

 disturbed their claws are brandished in an amusing manner, strikingly suggestive of 

 the motions of a violinist, whence these forms have received the common name of 

 " Fiddler Crabs." These crabs live in burrows in the ground near high-water mark, 

 salt marshes being a favorite resort. One of these forms constructs an oven-like arch 

 over the top of its hole, and there spends a large portion of his time, ready to descend 

 to his underground home at the approach of an enemy. Ocypoda has a somewhat 



