50 



became reduced, and a form was assumed which is more or 

 less nearly represented by the Zoaea stage, which occurs in 

 the development of most of the higher Crustacea. 



The Edriophthalmata must have sprung from the primi- 

 tive Malacostraca far back. They have no Zoaea stage, and 

 consequently may have diverged during that first Phyllopod 

 condition of the Malacostraca which Nehalia still represents, 

 or they may have arisen later on and have lost the Zoaea 

 stage in their development since. The Edriophthalmata 

 probably underwent a considerable amount of divergence 

 before breaking up into the groups now existing. The 

 Stomatopoda and Cumacea may be represented as short 

 lateral branches from the Malacostraca after the Zosea-like 

 ancestral stage. The Cumacea in some respects show 

 resemblances to the Edriophthalmata. 



The ancestral Podophthalmata then split into the Schizo- 

 poda, a small group which did not diverge much, and which 

 may be taken as representing an ancestral stage (Mysis), 

 and the Decapoda, which after some further evolution, 

 including the loss of the Schizopod character— the presence 

 of exopodites on the posterior thoracic appendages — became 

 broken up into the ancestral forms of the existing groups. 

 The Macrura and the Brachyura form two somewhat divergent 

 series, while the Anomoura have undergone considerable 

 modification or degeneration. Penceiis, amongst the Macrura, 

 has been shown by Fritz Miiller* to leave the egg as a 

 Nauplius, and to go through a series of larval stages, which 

 probably represent more nearly than any other forms the more 

 important ancestral stages in the phylogeny of the Crustacea. 



In the Decapoda the evolution of the sense organs, the 

 efficiency of the exoskeleton, the heteronomy of the segments, 

 and the specialization of the appendages reach their highest 

 degree of perfection. 



* Facts for Darwin, London, 1869. 



