BRANCHIOPODA. 



371 



by different authors as corresponding with the two branches 

 usually met with. By a comparison of the undoubtedly bira- 

 mous second antennae with the developing thoracic legs of the 

 larval Apus, Lankester * has shown good reason for regarding 

 the fifth and sixth (the two terminal) endites as the homo- 

 logues of the endopodite and exopodite 

 respectively. It follows that the 

 flabellum must be regarded, not as an 

 exopodite (its jointed flagellar termina- 

 tion in Limnetis notwithstanding) but 

 as a distal epipodite. 



In Daphnia among the Cladocera 

 (Fig. 252) the epipodites are reduced to 

 a single bract and the distal endites 

 are diminished in size. The coxal en- 

 dite on the other hand is large in the 

 first pair of thoracic legs, and in the 

 second and third pairs it forms a great 

 comblike, backwardly directed plate. 



In the Polyphemidae the anterior 

 or all the legs have a cylindrical 

 shape, and in Leptodora no epipodite is 

 present. 



It is remarkable that in Apus, which 

 possesses a large number of appendages (40-63 pairs), while the 

 anterior pairs correspond in number with the segments (as indi- 

 cated by the annular constrictions and rings of cuticular spines 

 on the surface of the body), being attached one to each, the 

 posterior far outnumber the segments which bear them, so that 

 one segment carries as many as five or six pairs of limbs. 



The central nervous system is composed of a supra- oesophageal 

 mass connected by commissures with a ventral chain of ganglia. 

 The latter in the Phyllopoda presents nearly the same simple 

 arrangement which is found in the Annelida, consisting of a 

 pair of ganglia to each pair of appendages (Fig. 241, D). There 

 are, however, in the Phyllopoda, two transverse commissures 

 to each pair of ganglia. Even Apus with its large number of 



* Q.J.M.S., vol. 21. The evidence here cited, and that brought for- 

 ward by Thiele (Betrachtungen iib. die. Phylogenie der Crustaceenbeine. 

 Zeit. /. wiss. Zoologie. Bd. 82 (1905) p. 445), who regards the flabellum 

 as the exopodite, appear to be conflicting. 



FIG. 249. Second thoracic leg of 

 Apus cancriformis. 1 first, 2 

 second stem joint ; En 1-6 

 endites ; ep flabellum ; ep' 

 bract (from Korschelt and 

 Heider, after Lankester). 



