504 PHYLOGENY OF THE CRUSTACEA. 



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 antennae 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 palaeontological and 

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

 that the later Nauplius stages of the different Entomostracan groups and 

 the Malacostraca (Penaeus 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, Penaeus 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 Protozoaea stage in which the maxillae 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 Zoaea stage the chief features of which have already been fully 

 characterised (vide p. 465). Three more or less distinct types of Zoaea are 

 distinguished by Claus. (a) That of Penaeus, 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 Zoaea 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 

 larvae. 



The embryological questions requiring to be settled concern the value 



