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? 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 much larger, and the most central of all the Crustacean groups; and which more nearly retains in the characters of the second pair of antennæ etc. the Nauplius peculiarities. This view is shared both by Claus and Dohrn, and appears to be in accordance with all the evidence we have both palæontological and morphological. Claus indeed carries this view still further, and believes that the later Nauplius stages of the different Entomostracan groups and the Malacostraca (Penæus 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.
It is beyond the scope of this work to attempt to grapple with all the difficulties which arise in connection with the origin and relationships of the various phyla, but I confine myself to a few suggestions arising out of the developmental histories recorded above.
Malacostraca. In attempting to reconstitute from the evidence in our possession the ancestral history of the Malacostraca we may omit from consideration the larval history of all those types which leave the egg in nearly the adult form, and confine our attention to those types in which the larval history is most completely preserved.
There are three forms which are of special value in this respect, viz. Euphausia, Penæus and Squilla. From the history of these which has already been given it appears that in the case of the Decapoda four stages (Claus) may be traced in the best preserved larval histories.
1. A Nauplius stage with the usual Nauplius characters.
2. A Protozoæa stage in which the maxillæ and first pair of maxillipeds are formed behind the Nauplius appendages; but in which the tail is still unsegmented. This stage is comparatively rarely preserved and usually not very well marked.
3. A Zoæa stage the chief features of which have already been fully characterised (vide p. [465]). Three more or less distinct types of Zoæa are distinguished by Claus. (a) That of Penæus, in which the appendages up to the third pair of maxillipeds are formed, and the thorax and abdomen are segmented, the former being however very short. The heart is oval, with one pair of ostia. From this type the Zoæa forms of the other Decapoda are believed by Claus to be derived. (b) That of Euphausia, with but one pair of maxillipeds and those short and Phyllopod-like. The heart oval with one pair of ostia. (c) That of Squilla, with an elongated many-chambered heart, two pairs of maxillipeds and the abdominal appendages in full activity.
4. A Mysis stage, which is only found in the macrurous Decapod larvæ.
The embryological questions requiring to be settled concern the value of the above stages. Do they represent stages in the actual evolution of the present types, or have their characters been secondarily acquired in larval life?
With reference to the first stage this question has already been discussed, and the conclusion arrived at, that the Nauplius does in a much modified form represent an ancestral type. As to the fourth stage there can be no doubt that it is also ancestral, considering that it is almost the repetition of an actually existing form.