26 THE CRUSTACEA 



more or less closely, the actual structure of ancestral types. As 

 regards the zoea, this assumption was soon shown to be erroneous, 

 and the secondary nature of this type of larva is now generally 

 admitted. The nauplius, however, by the constancy of its general 

 characters in the most widely diverse groups, shows itself to be 

 a very ancient type, and the view has been advocated that the 

 Crustacea have arisen from an unsegmented nauplius - like 

 ancestor. To this view there are considerable objections. Several 

 stnictures which can hardly have been absent from the common 

 stock of the Crustacea, such as the paired eyes and the shell-fold, 

 are not found in the nauplius. Other characters common to 

 certain Crustacea and Annelids, such as the mode of growth of 

 the somites, the structure of the nervous system and of the heart, 

 can hardly be supposed to have arisen independently in the two 

 groups. The view now most generally adopted is that the 

 Branchiopoda, and especially Ajms, which resemble the Annelids 

 in the characters just mentioned and also in the large number 

 and uniformity of the trunk-somites and their appendages, approach 

 most nearly to the primitive Crustacean type. On the other hand, 

 in some respects, such as the reduced mouth-parts, the Branchiopoda 

 are considerably specialised. In some Copepoda the cephalic 

 appendages are much more primitive than in the Branchiopoda, 

 and the first three pairs of appendages retain throughout life, with 

 little modification, the shape and function which they have in the 

 nauplius stage. It is possible, however, that in these characters 

 the Copepoda are persistently larval rather than phylogenetically 

 primitive, and in other respects, especially in the absence of paired 

 eyes and of a shell-fold, they are certainly specialised. 



In order to reconstruct the hypothetical ancestral type, there- 

 fore, it is necessary to combine the characters of several of the 

 existing groups. It may be supposed to have approximated, in 

 general form, to Apus, with an elongated body of numerous similar 

 somites, terminating in a caudal furca ; with the postoral appendages 

 all similar, and with a carapace originating as a shell-fold from the 

 maxillary region. The eyes were probably stalked and movable, the 

 antennules uniramous, and the antennae and mandibles biramous 

 and natatory, and both armed with masticatory processes. The 

 trunk -limbs were probably biramous but with additional endites 

 and exites, and all provided with gnathobases. 



It is to be noted that the Trilobita, which, according to the 

 classification adopted in this work, are dealt with under Arachnida, 

 approximate to the structure of the primitive Crustacean here 

 sketched except in the absence of a shell-fold and in having the 

 eyes sessile. 



It is not necessary, on this view, to deny all phylogenetic 

 significance to the nauplius. It may be regarded as an ancestral 



