506 PHYLOGENY OF THE CRUSTACEA. 



5. The presence of gill pouches (epipodites) on many of the append- 

 ages 1 . 



In addition to these points, to which others might be added, Claus 

 attempts to shew that Nebalia must be regarded as a type intermediate 

 between the Phyllopods and Malacostraca. This view seems fairly esta- 

 blished, and if true is conclusive in favour of the Phyllopod origin of the 

 Malacostraca. If the Protophyllopod origin of the Malacostraca is admitted, 

 it seems clear that the ancestral forms of the Malacostraca must have de- 

 veloped their segments regularly from before backwards, and been provided 

 with nearly similar appendages on all the segments. This however is far 

 from the case in existing Malacostraca, and Fritz Miiller commences his 

 summary of the characters of the Zoaea in the following words 2 . "The 

 middle body with its appendages, those five pairs of feet to which these 

 animals owe their name, is either entirely wanting or scarcely indicated." 

 This he regards as an ancestral character of the Malacostraca, and is of 

 opinion that their thorax is to be regarded as a later acquirement than the 

 head or abdomen. Claus' answer on this point is that in the most primitive 

 Zoaeae, viz. those already spoken of as types, the thoracic and abdominal 

 segments actually develop in regular succession from before backwards* 

 and he therefore concludes that the late development of the thorax in the 

 majority of Zoaea forms is secondary and not an ancestral Phyllopod 

 peculiarity. 



This is the main argument used by Claus against the Zoaea having any 

 ancestral meaning. His view as to the meaning of the Zoaea may be 

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

 existing Zoaea 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 Malacostracan groups, led secondarily 

 " to the three different Zoaea configurations to which probably later modifica- 

 " tions were added, as for instance in the young form of the Cumaceae. We 

 "might with the same justice conclude that adult Insects existed as cater- 

 " pillars or pupae as that the primitive form of the Malacostraca was a 

 " Protozoaea or Zoaea." 



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 

 Zoaea 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 



1 Claus appears to consider it doubtful whether the Malacostracan gills can be 

 compared with the Phyllopod gill-pouches. 

 * Facts for Darwin, p. 49. 



