CRUSTACEA. 50$ 



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. 



The second stage can clearly only be regarded as an embryonic prepara- 

 tion for the third ; and the great difficulty concerns the third stage. 



The natural view is that this stage like the others has an ancestral value, 

 and this view was originally put forward by Fritz Miiller and has been 

 argued for also by Dohrn. On the other hand the opposite side has been 

 taken by Claus, who has dealt with the question very ably and at great 

 length, and has clearly shewn that some of Fritz Miiller's positions are 

 untenable. Though Claus' opinion is entitled to very great weight, an 

 answer can perhaps be given to some of his objections. The view adopted 

 in this section can best be explained by setting forth the chief points which 

 Claus urges against Fritz Miiller's view. 



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

 traca 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 appear to me very conclusive, to shew that the Malacos- 

 traca are derived from a late Protophyllopod type, and Claus' 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. 



1. The compound eyes, so often stalked in both groups. 



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

 the Zoasa 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. 



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 Protophyl- 

 lopod type from which the Malacostraca originated as far more like existing Phyl- 

 lopods 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 Protozosea and earliest Cyclops stage. Yet if 

 I understand him rightly, he does not consider the Protozosea 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. 



