418 PHYLOGENY OF THE CRUSTACEA. 



it does not explain several other peculiarities of the Nauplius 1 . The more 

 important of these are the following. 



1. That the mandibles have the form of biramous swimming feet and 

 are provided with a cutting blade. 



2. That the second pair of aiiteiinse are biramous swimming feet 

 with a hook used in mastication, and are innervated (?) from the sub- 

 oesophageal ganglion. 



3. The absence of segmentation in the Nanplius body. An absence 

 which is the more striking in that before the Nauplius stage is fully 

 reached the body of the embryo is frequently divided into three segments, 

 e.g. Copepoda and Cirripedia. 



4. The absence of a heart. 



5. The presence of a median single eye as the sole organ of vision. 



Of these points the first, second, and fifth appear only to be capable 

 of being explained phylogenetically, while with reference to the absence 

 of a heart it appears very improbable that the ancestral Crustacea were 

 without a central organ of circulation. If the above positions are ac- 

 cepted the conclusion would seem to follow that in a certain sense the 

 Nauplius is an ancestral form but that, while it no doubt had its three 

 anterior pairs of appendages similar to those of existing Nauplii, it may 

 perhaps have been provided with a segmented body behind provided with 

 simple biramous appendages. A heart and cephalo-thoracic shield may also 

 have been present, though the existence of the latter is perhaps doubtful. 

 There was no doubt a median single eye, but it is difficult to decide whether 

 or no paired compound eyes were also present. The tail ended in a fork 

 between the prongs of which the anus opened; and the mouth was protected 

 by a large upper lip. In fact, it may very probably turn out that the most 

 primitive Crustacea more resembled an Apus larva at the moult immediately 

 before the appendages lose their Nauplius characters (fig. 208 B), or a 

 Cyclops larva just before the Cyclops stage (fig. 229), than the earliest 

 Nauplius of either of these forms. 



If the Nauplius ancestor thus reconstructed is admitted to have existed, 

 the next question in the phylogeny of the Crustacea concerns the relations 

 of the various phyla to the Nauplius. Are the different phyla descended 

 from the Nauplius direct, or have they branched at a later period from 

 some central stem 1 It is perhaps hardly possible as yet to give a full and 

 satisfactory answer to this question, which requires to be dealt with for each 

 separate phylum; but it may probably be safely maintained that the existing 

 Phyllopods are members of a group which was previously mucli larger, 

 and the most central of all the Crustacean groups ; and which more nearly 

 retains in the characters of the second pair of antennae etc. the Nauplius 

 peculiarities. This view is shared both by Glaus and Dohrn, and appears 

 to be in accordance with all the evidence we have both palreontological and 

 morphological. Glaus indeed carries this view still further, and believes 

 that the later Nauplius stages of the different Entoniostracan groups and 

 the Malacostraca (Peiiseus larva) exhibit undoubted Phyllopod affinities. 

 He therefore postulates the earlier existence of a Protophyllopod form, 

 which would correspond very closely with the Nauplius as reconstructed 

 above, from which he believes all the Crustacean groups to have diverged. 



1 For the characters of Nanplius vide p. 381. 



