i6o THE CRUSTACEA 



numerous. In most cases there are three such appendages, the 

 telson being produced as a median style between the furcal rami. 



Affinities and Classification. 



The alliance of the Leptostraca Avith the other ]\Ialacostraca is 

 amply justified by the agreement in the number of the appendages, 

 by the sharp distinction between the thoracic and abdominal series, 

 and by the position of the genital apertures. Other characters, 

 probably of less importance, are the biramous form of the anten- 

 nules, the possession of a masticatory stomach, and the complex 

 structure of the brain. The most important differences from the 

 Eumalacostraca are the presence of an additional somite in the 

 abdomen, of a caudal furca, and of an adductor muscle of the carapace. 



FlQ. 93. 



Ceratiocaris ■papilio. a, traces of antenniiles (?) ; in, toothed plates, possibly 

 the mandibles ; /•, rostal plate. (After H. Wooihvurd.) 



These are no doubt primitive features and indicate that the Lepto- 

 straca diverged from the Malacostracan stock before the assumption 

 of the typical caridoid form. It may be suggested that the 

 development of the uropods to form a tail-fan in the primitive 

 Eumalacostraca was associated with the loss of the caudal furca. 



The resemblance of the lamellar thoracic limbs of Nehalia to 

 those of the Branchiopoda, which has led to the Leptostraca being 

 associated in many classifications with that group, is doubtless 

 significant, and it becomes still more striking in the case of 

 Nebaliopsis. The absence of endites in the Leptostracan limb, 

 however, is an important difference. According to the view, 

 already mentioned (p. 42), that the exopodite is represented, in 

 the appendages of A-pns, for example, not by the flabellum but by 

 the sixth endite, it would seem impossible to draw a close com- 

 parison with the appendages of Nehalia. Without going so far 

 as this, however, it may be suggested as a possibility that the 



