THE BRANCHIOPODA 51 



Many palaeozoic fossils formerly classed with the " Phyllopoda " 

 are now referred to the Phyllocarida. 



AFFINITIES AND CLASSIFICATION. 



The alliance of the groups included in the Branchiopoda is 

 justified especially by the lobed foliaceous form of the trunk-limbs 

 which they have in common, but the divergences of structure in 

 other respects are greater than is the case in other sub-classes of 

 Crustacea. Thus, while the other sub -classes are more or less 

 strictly nomomeristic, each of the orders of Branchiopoda, and even 

 some of the families and genera, are markedly anomomeristic. This 

 is in agreement with the view that the Branchiopoda are a primitive 

 group which has not attained to the fixity of general structure 

 found in the other sub-classes. 



Their primitive character is further shown, as has been pointed 

 out, by the general uniformity of the trunk -somites and their 

 appendages, by the presence of gnathobases on all the trunk-limbs, 

 by the " ladder-like " form of the ventral nerve-chain and the post- 

 oral position of the antennal ganglia, and by the tubular heart and 

 its segmentally arranged ostia. The primitive character of the 

 larval development has also been alluded to. 



It may be mentioned here that, as in other groups of Arthro- 

 poda, the possession by many Branchiopoda of a large number of 

 somites can hardly be regarded as proof of their primitive position. 

 In the Notostraca, the fact that the posterior pairs of appendages 

 exceed in number the somites which carry them, shows that 

 secondary changes, whether by coalescence of somites or, more 

 probably, by multiplication of appendages, have taken place. A 

 further argument in favour of a possible increase in number of 

 somites is afforded by a consideration of the aberrant Notostracan 

 genus Polyartemia (Fig. 32). Apart from the Cladocera, which the 

 abbreviation of the trunk excludes from the comparison, Polyartemia 

 forms the only exception to the rule that the genital apertures of 

 the Branchiopoda are situated, approximately, in the region of the 

 twelfth trunk-somite. Now, the close resemblance in all other 

 respects between Polyartemia and the other Anostraca strongly 

 suggests that the nineteen somites interposed between the head 

 and the genital somite in that genus correspond, as a whole, to the 

 eleven somites which occupy the same position in the other 

 Anostraca; and the agreement of the latter in this respect with 

 most of the other Branchiopoda seems to indicate that the smaller 

 number of somites is here the more primitive, the larger the more 

 specialised condition. If this be so, a similar multiplication of 

 somites in the post-genital region of Notostraca may well account 

 for the exceptionally large number found in that order. 



