THE DEC APOD A 255 



species of fossil Decapoda have been little studied from the point of 

 view of phylogeny, but reference may be made to Bouvier's essay 

 on the origin of the Brachyura as an example of the results which 

 may be obtained in tliis department. Among other papers whicli 

 have been fruitful in suggesting lines of research for later workers 

 may be mentioned Huxley's memoir on the classification and distri- 

 bution of the Crayfishes (1878); A. Milne-Edwards's note on the 

 transformation of the ocular peduncle into an antenna-like organ in 

 a Palinurid (18G4), the forerunner of much recent work on regenera- 

 tion and abnormalities ; Giard's papers on pai'asitic castration ; and 

 Faxon's discovery of the alternating dimorphism in the males of 

 Camharus. 



Morphology. 



Amid the great diversity of general shape exhibited l\y the 

 Decapoda, two chief types may be distinguished. In the fii'st or 

 Macrurous type the general caridoid facies is retained, the body is 

 elongated and subcylindrical, the abdomen is long and terminates in 

 a tail-fan. In the Brachyurous type (which is not confined to the 

 Brachyura, but recurs in several groups of Anomura) the cephalo- 

 thorax is greatly expanded laterally and more or less depressed, 

 while the abdomen is reduced and folded underneath the cephalo- 

 thorax. A very peculiar modification is found in most Paguridea, 

 where the abdomen is markedly asymmetrical and spirally coiled, 

 in correlation with the habit of living in the empty shells of 

 Gasteropod Molluscs. 



The carapace coalesces dorsally with all the thoracic somites 

 and overhangs on each side as a hranchiostegite, enclosing the 

 branchial chamber Avithin which the gills are concealed. Anteriorly 

 it may be produced into a rostrum, which in a few genera of Caridea 

 (Iiht/nchocineles, etc.) is movably articulated. In most Brach^'ura 

 the rostrum is i-educed to a short but broad frontal plate, of which 

 tlie relations to the adjacent parts will be described below. In 

 some Macrura (Scyllaridae) and in many Anomura and Brachyura, 

 where the cephalothorax is flattened from above downwards, the 

 lateral portions of the carapace are abruptly bent inwards towards 

 the bases of the legs. The lateral margin thus produced is 

 commonly toothed or otherwise armed. 



The surface of the carapace is commonly marked by depressions 

 and grooves corresponding in part to the insertions of various muscles, 

 but in part independent of these. In this way several regions 

 of the carapace are defined which, especially in the Brachyura, 

 may be still further divided into sub-regions. For convenience 

 of systematic description these various areas arc denominated ac- 

 cording to a scheme of terminology introduced for the most part 

 by H. Milne-Edwards (1851). More recently the furroAvs of the 



