CRUSTACEA. 507 



imperfect and provided only with delicate foliaceous appendages not pro- 

 jecting 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 Zoaea stage in which the thoracic appendages 

 were completely aborted. Claus maintains that the Zoaea 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 Zoaea pecu- 

 liarities 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 corre- 

 sponding to the third pair of maxillipeds and first two pairs of ambulatory 

 appendages of the Decapoda are developed in the Protozoaea stage, but 

 completely aborted in the Zoaea stage, and subsequently redeveloped. 



2. In the case of the larva of Sergestes in the passage from the Acan- 

 thosoma (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. Clans' 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 Zoaea forms, its actual atrophy in the Penaeus Zoasa, and its 



