xi PHYLUM ARTHROPODA 525 



The indefinite number of segments, varying considerably in the 

 species of both genera, and the presence of a shell-gland place them 

 among the Entomostraca. 



The leaf-like character of the swimming-feet is alone sufficient 

 to assign them to the Phyllopoda, and the large number (consider- 

 ably more than ten) of segments and swimming-feet decides their 

 position among the Euphyllopoda. 



They are placed in the family Apodidse in virtue of the elongated 

 body with 40-60 pairs of swimming-feet, diminishing in size from 

 before backwards, and showing considerable differentiation ; and of 

 the elongated heart reaching to the twelfth post-cephalic segment. 



Apus is distinguished by the absence of a post-anal plate, and 

 by elongated flagella (endites) to the first pair of thoracic feet : in 

 Lepidurus the post-anal plate is present, and the flagella of the first 

 thoracic feet are short. 



Astacus fluviatilis is one of several species of the genus Astacus, 

 belonging to the family Potamobiidce, tribe Astacoidea, sub-order 

 Macrura, order Decapoda, and sub-class Malacostraca. 



The possession of twenty-one segments i.e. a prostomium, nine- 

 teen metameres, and a telson places it among the Malacostraca : 

 the presence of five pairs of thoracic legs without exopodites, and 

 of thoracic gills, among the Decapoda. 



The possession of a squame to the antenna, and of legs having all 

 seven podomeres distinct the first three pairs chelate, and the first 

 pair greatly enlarged determine its position in the tribe Astacoidea, 

 which includes all the fresh-water Crayfishes and the true Lobsters. 

 The family Potamobiidas is distinguished by having the podo- 

 branchiaB partly united to the epipodites, and by having append- 

 ages on the first abdominal segment of the male, and usually 

 on that of the female. 



3. GENERAL ORGANISATION. 



There is no class in the animal kingdom which presents so wide 

 a range of organisation as the Crustacea, or in which the devi- 

 ations in structure from the " type-form " are so striking and so 

 interesting from their obvious adaptation to the mode of life. 



The most interesting modifications are those connected with 

 the external characters and the structure of the append- 

 ages. As we have seen, the body consists of a prostornium, a 

 variable number of metameres, and an anal segment. The first 

 five metameres or four if the antennulary region is not reckoned 

 as a metamere fuse with the prostomium to form a head, which, 

 as well as the anal segment, is homologous throughout the class. 

 On the other hand, there is no strict homology between the 

 various post-cephalic metameres in different forms until we come 

 to the Malacostraca, in which their number is constant. 



