420 PHYLOGENY OF THE CRUSTACEA. 



The primary question which needs to be settled is whether the Mala- 

 costraca have diverged very early from the Nauplius root, or later in the 

 history of the Crustacea from the Phyllopod stem. On this question 

 Claus 1 brings arguments, which appeal* to me very conclusive, to shew 

 that the Malacostraca are derived from a late Protophyllopod type, and 

 Glaus' view on this point is shared also by Dohrn. The Phyllopoda present 

 so many characters (not possessed by the Nauplius) in common with the 

 Malacostraca or their larval forms, that it is incredible that the whole of 

 these should have originated independently in the two groups. The more 

 important of these characters are the following. 



"I. The compound eyes, so often stalked in both groups. 



2. The absence of a palp on the mandible, a very marked character of 

 the Zosea as well as of the Phyllopoda. 



3. The presence of a pair of frontal sense knobs. 



4. The Phyllopod character of many of the appendages. Cf. first pair 

 of maxillipeds of the Euphausia Zosea. 



5. The presence of gill pouches (epipodites) on many of the appendages 2 . 

 In addition to these points, to which others might be added, Claus 



attempts to shew that Nebalia must be regarded as a type intermediate 

 between the Phyllopods and Malacostraca. This view seems fairly esta- 

 blished, and if true is conclusive in favour of the Phyllopod origin of the Ma- 

 lacostraca. If the Protophyllopod origin of the Malacostraca is admitted, 

 it seems clear that the ancestral forms of the Malacostraca nmst have de- 

 veloped their segments regularly from before backwards, and been provided 

 with nearly similar appendages on all the segments. This however is far 

 from the case in existing Malacostraca, and Fritz Miiller commences his 

 summary of the characters of the Zotea in the following words 3 . "The 

 middle body with its appendages, those five pairs of feet to which these 

 animals owe their name, is either entirely wanting or scarcely indicated." 

 This he regards as an ancestral character of the Malacostraca, and is of 

 opinion that their thorax is to be regarded as a later acquirement than 

 the head or abdomen. Claus' answer on this point is that in the most 

 primitive Zosepe, viz. those already spoken of as types, the thoracic and 

 abdominal segments actually develop in regular succession from before 

 backwards, and he therefore concludes that the late development of the 

 thorax in the majority of Zoa3a forms is secondary and not an ancestral 

 Phyllopod peculiarity. 



This is the main argument used by Claus against the Zoa?a having any 

 ancestral meaning. His view as to the meaning of the Zoyea may be 



1 Claus speaks of the various Crustacean phyla as having sprung from a Protophyl- 

 lopod form, and it might be supposed that he considered that they all diverged from 

 the same form. It is clear however from the context that he regards the Protophyllopod 

 type from which the Malacostraca originated as far more like existing Phyllopods than 

 that from which the Entomostracan groups have sprung. It is not quite easy to get 

 a consistent view of his position on the question, since he states (p. 77) that the 

 Malacostraca and the Copepods diverged from a similar form, which is represented in 

 their respective developments by the Protozoaea and earliest Cyclops stage. Yet if 

 I understand him rightly, he does not consider the Protozoaaa stage to be the Proto- 

 phyllopod stage from which the Malacostraca have diverged, but states on p. 71 that 

 it was not an ancestral form at all. 



2 Claus appears to consider it doubtful whether the Malacostracan gills can be 

 compared with the Phyllopod gill-pouches. 



3 Facts for Darwin, p. 4'J. 



