Granting Claus’ two main positions, viz. that the Malacostraca are derived from Protophyllopods, and that the segments were in the primary ancestral forms developed from before backwards, it does not appear impossible that a secondary and later ancestral form may have existed with a reduced thorax. This reduction may only have been partial, so that the Zoæa ancestor would have had the following form. A large cephalo-thorax and well-developed tail (?) with swimming appendages. The appendages up to the second pair of maxillipeds fully developed, but the thorax very imperfect and provided only with delicate foliaceous appendages not projecting beyond the edge of the cephalo-thoracic shield.
Another hypothesis for which there is perhaps still more to be said is that there was a true ancestral Zoæa stage in which the thoracic appendages were completely aborted. Claus maintains that the Zoæa form with aborted thorax is only a larval form; but he would probably admit that its larval characters were acquired to enable the larva to swim better. If this much be admitted it is not easy to see why an actual member of the ancestral series of Crustacea should not have developed the Zoæa peculiarities when the mud-dwelling habits of the Phyllopod ancestors were abandoned, and a swimming mode of life adopted. This view, which involves the supposition that the five (or six including the third maxillipeds) thoracic appendages were lost in the adult (for they may be supposed to have been retained in the larva) for a series of generations, and reappeared again in the adult condition, at a later period, may at first sight appear very improbable, but there are, especially in the larval history of the Stomatopoda, some actual facts which receive their most plausible explanation on this hypothesis.
These facts consist in cases of the actual loss of appendages during development, and their subsequent reappearance. The two most striking cases are the following.
1. In the Erichthus form of the Squilla larva the appendages corresponding to the third pair of maxillipeds and first two pairs of ambulatory appendages of the Decapoda are developed in the Protozoæa stage, but completely aborted in the Zoæa stage, and subsequently redeveloped.
2. In the case of the larva of Sergestes in the passage from the Acanthosoma (Mysis) stage to the Mastigopus stage the two hindermost thoracic appendages become atrophied and redevelop again later.
Both of these cases clearly fit in very well with the view that there was an actual period in the history of the Malacostraca in which the ancestors of the present forms were without the appendages which are aborted and redeveloped again in these larval forms. Claus’ hypothesis affords no explanation of these remarkable cases.
It is however always possible to maintain that the loss and reappearance of the appendages in these cases may have no ancestral meaning; and the abortion of the first pair of maxillipeds and reduction of some of the other appendages in the case of the Loricata is in favour of this explanation. Similar examples of the abortion and reappearance of appendages, which cannot be explained in the way attempted above, are afforded by the Mites and also by the Insects, e.g. Bees.
On the other hand there is almost a conclusive indication that the loss of the appendages in Sergestes has really the meaning assigned to it, in that in the allied genius Leucifer the two appendages in question are actually absent in the adult, so that the stage with these appendages absent is permanently retained in an adult form. In the absence of the mandibular palp in all the Zoæa forms, its actual atrophy in the Penæus Zoæa, and its universal reappearance in adult Malacostraca, are cases which tell in favour of the above explanation. The mandibular palp is permanently absent in Phyllopods, which clearly shews that its absence in the Zoæa stage is due to the retention of an ancestral peculiarity, and that its reappearance in the adult forms was a late occurrence in the Malacostracan history.
The chief obvious difficulty of this view is the redevelopment of the thoracic feet after their disappearance for a certain number of generations. The possibility of such an occurrence appears to me however clearly demonstrated by the case of the mandibular palp, which has undoubtedly been reacquired by the Malacostraca, and by the case of the two last thoracic appendages of Sergestes just mentioned. The above difficulty may be diminished if we suppose that the larvæ of the Zoæa ancestors always developed the appendages in question. Such appendages might first only partially atrophy in a particular Zoæa form and then gradually come to be functional again; so that, as a form with functional thoracic limbs came to be developed out of the Zoæa, we should find in the larval history of this form that the limbs were developed in the pre-zoæal larval stages, partially atrophied in the Zoæa stage, and redeveloped in the adult. From this condition it would not be difficult to pass to a further one in which the development of the thoracic limbs became deferred till after the Zoæa stage.
The general arguments in favour of a Zoæa ancestor with partially or completely aborted thoracic appendages having actually existed in the past appear to me very powerful. In all the Malacostracan groups in which the larva leaves the egg in an imperfect form a true Zoæa stage is found. That the forms of these Zoææ should differ considerably is only what might be expected, considering that they lead a free existence and are liable to be acted upon by natural selection, and it is probable that none of those at present existing closely resemble the ancestral form. The spines from their carapace, which vary so much, were probably originally developed, as suggested by Fritz Müller, as a means of defence. The simplicity of the heart—so different from that of Phyllopods—in most forms of Zoæa is a difficulty, but the reduction in the length of the heart may very probably be a secondary modification; the primitive condition being retained in the Squilla Zoæa. In any case this difficulty is not greater on the hypothesis of the Zoæa being an ancestral form, than on that of its being a purely larval one.