148 THE CRUSTACEA 



An important departure from the line of classification generally 

 followed was made in 1883 by Boas, who showed that the order 

 " Schizopoda " as then understood comprised two very diflierent 

 groups, which he separated as distinct orders, the Euphausiacea and 

 Mysidacea. Boas discarded the old divisions Podo2:)hthalma and 

 Edriophthalma, and divided the Malacostraca into seven orders, 

 Euphausiacea, Mysidacea, Cumacea, Isopoda, Amphipoda, Decapoda, 

 and Squillacea (or Stomatopoda). Hansen, in 1893, carried the 

 reform of the classification a step further. Setting apart (as 

 Huxley had previously done) the aberrant Stomatopoda, as well 

 as the Leptostraca, he showed that the remaining Malacostraca 

 fell into two well-defined groups, the line of division passing 

 through the old order Schizopoda ; on the one side he placed the 

 Euphausiacea with the Decapoda, and on the other the Mysidacea 

 with the Cumacea and the Edriophthalmate orders Tanaidacea, 

 Isopoda, and Amphipoda. The classification adopted here is 

 essentially that of Hansen, as modified and extended by the 

 present writer in 1904. 



It will be convenient to give here definitions of the main 

 groups into which the Malacostraca are divided. The orders Avill 

 be considered in greater detail in the subsequent chapters. 



Sub-Class Malacostraca. 

 Series I. Leptostraca, Glaus (1880). 



Abdomen of seven somites, the last of which is without appendages, 

 and a telson bearing a pair of movably articulated furcal rami ; an 

 adductor muscle runs between the two valves of the carapace ; thoracic 

 limbs all similar, more or less foliaceous, with protopodite of three 

 segments. 



Series II. Eumalacostraca, Grobben (1892). 



Abdomen of six somites (the number may be reduced by coalescence), 

 the last of which typically bears a pair of appendages, and a telson which 

 never bears movable furcal rami ; no adductor muscle of the carapace ; 

 thoracic limbs rarely all similar (Euphausiacea), typically pediform, 

 protopodite of two segments except in Stomatopoda. 



Division 1. Syncarida, Packard (1 



Carapace absent ; first thoracic somite fused with the head or defined 

 therefrom by a groove ; protopodite of antenna of two segments ; mandible 

 without lacinia mobilis ; thoracic legs flexed between fifth and sixth 

 segments ; no oostegites ; no appendix interna on pleopods ; hepatic 

 caeca numerous ; heart much elongated, tubular. • 



