422 PHYLOGENY OF THE CRUSTACEA. 



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 genus 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 Zosea forms, its actual atrophy in the Peuseus 

 Zoaea, and its universal reappearance in adult Malacostraca, are cases which 

 tell in favour of the above explanation. The mandibular palp is per- 

 manently absent in Phyllopods, which clearly shews that its absence in 

 the Zoaea stage is due to the retention of an ancestral peculiarity, and 

 that its reappearance in the adult forms was a late occurrence in the 

 Malacostracau 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 demon- 

 strated 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 larvae of the Zosea ancestors always 

 developed the appendages in question. Such appendages might first only 

 partially atrophy in a particular Zoaaa 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 Zosea, we should find in the larval history 

 of this form that the limbs were developed in the pre-zoseal larval stages, 

 partially atrophied in the Zosea 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 Zosea 

 stage. 



The general arguments in favour of a Zosea 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 Zorea stage is found. 

 That the forms of these Zosese 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 Miiller, as a means of defence. The 

 simplicity of the heart so different from that of Phyllopods in most 

 forms of Zoa?a 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 Zoaea. In. any case this difficulty is not 

 greater on the hypothesis of the Zoaea being an ancestral form, than on that 

 of its being a purely larval one. 



