LARVAE. CLASSIFICATION. 453 



It may be replied that we are no further from an explanation 

 of the characters of these larvae than we are from an explanation 

 of the characters of the species and genera of many other living 

 things ; and this is doubtless true, but in offering the phylo- 

 genetic explanation of the mysis larva the endeavour is made 

 to discriminate among the possible factors from which its char- 

 acters result, and in doing so it seems well to point out the 

 difficulties with which the problem is beset. 



Whatever view we may take of the full significance of the 

 nauplius and zoaea larvae, it appears that the nauplius, from its 

 prevalence throughout the Crustacea, was established as a larval 

 form at a period of development prior to the divergence of the 

 existing groups, and the zoaea at an early stage in the history 

 of the Malacostraca. 



In certain respects the nauplius must be regarded as presenting 

 primitive features of the crustacean stock, and especially in 

 the paroral position of the second antennae, the presence of a 

 masticatory endite at the base of this appendage, and the large 

 biramous palp of the mandible. That the segment bearing the 

 second antennae was originally postoral is indicated in the develop- 

 ment of many groups of the Arthropoda. This was apparently 

 the adult condition in the Trilobites, and the nauplius retains 

 this as a character of a free-living stage (cf. p. 356). The biramous 

 character of the mandible persists in some Copepods (p. 396), 

 and Ostracods (p. 384). The median eye and the frontal sense 

 organs are also probably to be regarded as primitive features of 

 the nauplius stage. 



Of late years the unity of the group Schizopoda has been 

 challenged by Boas and Hansen, and the reasons for its partition 

 have recently been strongly urged by Caiman.* It is pointed 

 out that the characters which the two tribes of Schizopods 

 possess in common, and which distinguish them from the Deca- 

 pods are few in number, and of doubtful value. The modification 

 of the three anterior pairs of thoracic appendages as maxillipeds 

 distinguishes the great majority of the Decapods, but among 

 the Penaeidae it is little marked, and it is hard to draw a dis- 



* W. T. Caiman, On the Classification of the Crustacea Malacostraca, 

 Ann. and Mag. of N. H., ser. 7, vol. xiii (1906), p. 144. 



