PODOPHTHALMIA. 



59 



IG. 70. — Petrolisthes armatus, 

 porcelain crab, natural size. 



The LiTHODiD^, ■which in their form of body closely resemble the Maioid crabs, 

 to be mentioned further on, are represented on our East coast by two species, the 

 family acquiring its greatest development on the West coast of both North and South 

 America. The abdomen is without caudal appendages, and at first sight it would 

 appear that the name Decapods was a misnomer, as only four pairs of thoracic feet are 

 externally visible, but a little investigation shows that the fifth pair are present, though 

 folded up under the oarapax. Of the habits scarcely anything is known. 



The PoRCELLANiDiE are small, brightly-colored crabs, 

 with a shell always kept clean, and from its general re- 

 semblance to porcelain giving the name to the principal 

 genus. In these, as in the family just mentioned, the fifth 

 pair of legs are not used in locomotion, being carried 

 folded upon the back. The species, which are numerous 

 in the warmer seas of the world, live under rocks or among 

 the coral reefs. These forms are worthy of notice from a 

 morphological standpoint, for it would seem that here the 

 appendages of the seventh abdominal segment were par- 

 tially developed, showing the validity of its recognition as 



one of the body segments. 



The HippiD^ have an elongate body, and feet fitted 

 for swimming, while the way in which the abdomen is 

 bent also fits it for burrowing. The following account 

 of the habits of the American Hippa emerita is con- 

 densed from that furnished by Professor Smith : " This 

 species prefers a narrow zone of sandy beaches, near 

 low-water mark, where it lives gregariously, burrowing 

 beneath the surface. They burrow with the greatest 

 rapidity, and always backward, pushing themselves in 

 by means of their thoracic feet. In an aquarium they 

 at once plunged entirely beneath the sand, and then 

 in an upright position showed just the tips of the an- 

 tennulsB and the eyes at the surface. Of the food but 

 little can be said. In all the specimens examined the 

 alimentary canal was filled with fine sand, nearly free 

 from organic matter, though under the microscope a 

 small amount of vegetable matter is seen, rendering it 

 probable that the sand is swallowed for the 

 nutritive matter it may contain." 



The TELEOSOMI differ from the group 

 just mentioned in having the last thoracic 

 segment anchylosed to the others, as in most 

 Decapoda, while the outlets of the female 

 genital organs occupy the same position as 

 in the group just mentioned. The first 

 form we will notice is Hypoconcha, the 

 "false hermit" of the older writers, of which 

 three species are known in tropical Ameri- 

 ' can waters. In these forms the dorsal surface of the carapax is soft, resembling in 



Fig. 71. —Bippa emerita. 



Fig. 72. — Dromia. 



