ORIGIN AND RADIATION 17 



the Reptantia, with over 4,400 species spread throughout the world. 

 They are characterised by the small abdomen tucked forwards 

 under the body, and by the lack of a tail fan. One group, the 

 Dromiacea, still has remants of a tail fan, and is regarded as the 

 most primitive group of the Brachyura. This opinion is supported 

 by the fossil record; the Dromiacea were the dominant crabs during 

 the Jurassic, before other crabs had evolved. Some authorities 

 regard the Dromiacea as too primitive to be included in the 

 Brachyura and would put them in a group on their own. 



The Anomura show many features which are intermediate 

 between those of true crabs and lobsters, as well as having their 

 own specialisations. Some, such as Galathea (fig. 15) carry the 

 abdomen flexed forwards beneath the thorax, but it is still capable 



Fig. 15. Galathea squamifera (Decapoda, Reptantia), an anomuran 



Notice the small last leg tucked in between the abdomen and the 



thorax. Actual length of body in position shown about 3 cm. 



of complete extension backwards. The hermit crabs have large soft 

 abdomens which they keep covered with the empty shells of various 

 molluscs. In connection with this peculiar habit they have lost the 

 appendages on the right side of the abdomen. This anatomical 

 peculiarity gives a clue to the ancestry of the Robber Crab, 

 Birgus latro, which has a hard symmetrical abdomen, but only has 

 pleopods on the left side. The most reasonable explanation is that 

 the Robber Crab has descended from hermit crabs which gave up 

 carrying shells and redeveloped a hard abdomen, but could not 

 regain their lost appendages. 



Some of the Anomura, such as the porcelain crabs (family 

 Porcellanidae), look like true crabs, but the small abdomen has a 

 distinct tail fan, and the last pair of legs are small and tucked into 

 the space between the carapace and the abdomen, leaving the 



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