THE DECAPODA 309 



AFFINITIES AND CLASSIFICATION. 



The resemblances between the lower, Decapoda, especially the 

 Penaeidea, and the Euphausiacea have been mentioned in dealing 

 with the .latter Order, and justify the alliance of the two Orders in 

 the Division Eucarida. It may be mentioned that the exopodites 

 of the thoracic legs, the absence of Avhich still survives in text-books 

 as distinguishing the Decapoda from the " Schizopoda," are at least 

 as strongly developed in many Caridea (Hoplophoridae, etc.) (Fig. 

 160, p. 270) as in Euphausiacea or Mysidacea. Coutiere has recently 

 called attention to some curious resemblances between certain 

 primitive Caridea and the Lophogastrid Mysidacea. These 

 resemblances, however, by no means outweigh the important 

 differences between the two groups, and may be either primitive 

 characters derived from the common caridoid stock or convergences 

 due to similarity of habits. 



The classification of the Decapoda is a very difficult problem, 

 and none of the schemes hitherto proposed can be regarded as 

 entirely satisfactor}^ The traditional classification of the group 

 into the long-tailed Macrura and short-tailed Brachyura was estab- 

 lished by Latreille in 1806 ; but the difficulty of defining these 

 groups is shown by the varying limits which have been assigned to 

 the intermediate group of Anomura established by Milne-Edwards 

 in 1834. Boas, in 1880, was the first to make a radical departure 

 from this system. He pointed out that the Brachyura and Anomura 

 were only single branches of the Decapod stock, and by no means 

 equal in systematic value to the Macrura, which included several 

 other branches not more closely connected with each other. In 

 other words, just as in the classification of the Malacostraca as a 

 whole, so within the Order Decapoda, the retention of the primitive 

 " caridoid facies " does not necessarily imply close affinity between 

 the groups exhibiting it. Boas proposed a division of the Order 

 into the two primary groups of Natantia and Reptantia as defined 

 below. This division is undoubtedly a more natural one than those 

 formerly employed, although it is hardly more easy to find constant 

 and exclusive structural characters by which to define the sub-orders 

 than it was in the case of the Macrura, Anomura, and Brachyura. 

 A further difficulty is presented by the small group of Stenopidea, 

 which combine, to some extent, the characters of Natantia and 

 Reptantia, and may perhaps deserve separation as a third sub-order. 

 Important modifications of Boas's scheme have been introduced by 

 Ortmann and by Borradaile, and the classification of the last-named 

 author has been adopted here, with some alterations, chiefly of a 

 formal kind, as, on the whole, the most satisfactory yet proposed. 

 Borradaile's chief innovations are the inclusion of the Thalassinidea, 



