A STUDY IN MORPHOLOGY. 11'.) 



Malacostraca by actual moults, will not only help us to a sound and thorough appre- 

 ciation of the significance of Crustacean embryology, but will also contribute to a 

 better knowledge of the relation between ontogeny and phytogeny in the whole 

 province of biology. 



The phylogenetic significance of the Xauplius stage of development seems to me to 

 rest upon a much firmer basis, and there are many reasons for believing that this is 

 really an ancestral form. Its occurrence in so many widely-separated groups of 

 Crustacea shows its great antiquity, and if it does not represent the adult form of the 

 ancestral Crustacea, but a later larval form which has been produced by secondary 

 modification of the original course of development, this secondary modification must 

 have taken place very early in the history of the group, at a time when the adult 

 forms were very primitive and unspecialised. A sufficient difference between the 

 habits and surroundings of a young animal and those of the adult to favour secondary 

 modification of the young is much less probable in an early unspecialised form, with 

 simple habits, than it is in later and higher forms ; and the older a larval form can 

 be shown to be, the more probable does it become that it at one time existed as an 

 adult. 



The great age of the Naiqrfius stage and its definite structure therefore indicate 

 that it is ancestral, and nothing except the supposed necessity for believing that the 

 primitive Crustacean had a great number of somites and appendages seems to oppose 

 this view. 



I shall try to show further on that the serial homology shown by the parts of the 

 body of one of the higher Crustacea cannot be fully accounted for by assuming, with 

 BALFOUR ('Comparative Embryology,' p. 418), that the primitive Crustacean had, in 

 addition to its three pairs of appendages similar to those of existing Nauplii, a long 

 segmented body with simple biramous appendages ; and I shall also try to show that 

 this homology can be accounted for without any such supposition, so that the 

 peculiarities which BALFOUR points out 1st, that the mandibles have the form of 

 biramous swimming feet ; 2nd, that the second pair of antenna? are biramous swim- 

 ming feet ; 3rd, that the body shows no traces of segmentation ; 4th, that the heart is 

 absent ; 5th, that the ocellus is the sole organ of vision must be allowed their full 

 weight, and must not be opposed by any d priori assumption of the theoretical need 

 for a greater number of somites and appendages. 



