CRUSTACEA. 421 



gathered from the following passage. After assuming that none of the 

 existing Zosea types could have been adult animals, he says " Much more 

 " probably the process of alteration of the metamorphosis, which the Mala- 

 "costracan phylum underwent in the course of time and in conjunction 

 "with the divergence of the later Malacostracaii groups, led secondarily 

 "to the three different Zosea configurations to which probably later 

 " modifications were added, as for instance in the young form of the 

 " Cumacefe. We might with the same justice conclude that adult Insects 

 " existed as caterpillars or pupa? as that the primitive form of the Malaco- 

 " straca was a Protozosea or Zosea,." 



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 im- 

 possible that a secondary and later ancestral form may have existed with 

 a reduced thorax. This reduction may only have been partial, so that the 

 Zosea ancestor would have had the following form. A large cephalo-thorax 

 and well-developed tail (I) 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 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 Zosea stage in which the thoracic 

 appendages were completely aborted. Claus maintains that the Zosea 

 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 Zosea 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 Stornatopoda, 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 Protozosea stage, but 

 completely aborted in the Zosea 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 veiy 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. 



