508 PHYLOGENY OF THE CRUSTACEA. 



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 Zoasa 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 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 Zoaea ancestors always 

 developed the appendages in question. Such appendages might first only 

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

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

 partially atrophied in the Zoaea 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 Zoaea stage. 



The general arguments in favour of a Zoaea 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 Zoaea stage is found. 

 That the forms of these Zoaeae 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 Zoaea 

 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. 



The points of agreement in the number and character of the appendages, 

 form of the abdomen, etc. between the various types of Zoaea appear to me 

 too striking to be explained in the manner attempted by Claus. It seems 

 improbable that a peculiarity of form acquired by the larva of some ancestral 

 Malacostracan should have been retained so permanently in so many groups 1 



1 A secondary larval form is less likely to be repeated in development than an 

 ancestral adult stage, because there is always a strong tendency for the former, which 

 is a secondarily intercalated link in the chain, to drop out by the occurrence of a 

 reversion to the original type of development. 



