119 
A STUDY IN MORPHOLOGY. 
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 phylogeny in the whole 
province of biology. 
The phylogenetic significance of the Nauplius 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 ad alt 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 Nauplius 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 antennae 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 a priori assumption of the theoretical need 
for a greater number of somites and appendages. 
