The second stage can clearly only be regarded as an embryonic preparation 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 Müller 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 Müller’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 Müller’s view.
The primary question which needs to be settled is whether the Malacostraca have diverged very early from the Nauplius root, or later in the history of the Crustacea from the Phyllopod stem. On this question Claus[204] brings arguments, which appear to me very conclusive, to shew that the Malacostraca 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 Zoæa 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 Zoæa.
5. The presence of gill pouches (epipodites) on many of the appendages[205].
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 established, and if true is conclusive in favour of the Phyllopod origin of the Malacostraca. If the Protophyllopod origin of the Malacostraca is admitted, it seems clear that the ancestral forms of the Malacostraca must have developed 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 Müller commences his summary of the characters of the Zoæa in the following words[206]. “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 Zoææ, 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 Zoæa forms is secondary and not an ancestral Phyllopod peculiarity.
This is the main argument used by Claus against the Zoæa having any ancestral meaning. His view as to the meaning of the Zoæa may be gathered from the following passage. After assuming that none of the existing Zoæa types could have been adult animals, he says—“Much more probably the process of alteration of the metamorphosis, which the Malacostracan phylum underwent in the course of time and in conjunction with the divergence of the later Malacostracan groups, led secondarily to the three different Zoæa configurations to which probably later modifications were added, as for instance in the young form of the Cumaceæ. We might with the same justice conclude that adult Insects existed as caterpillars or pupæ as that the primitive form of the Malacostraca was a Protozoæa or Zoæa.”