570 ZOOLOGY SECT. 



ments (considerabl}' more than ten) and swimming-feet, together 

 with the presence of the large shield-like carapace, decides their 

 position among the Notostraca. 



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

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

 before backwards and showing considerable differentiation ; and of 

 the elongated hoart 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 fiuviatilis is one of several species of the genus Astacus, 

 belonging to the family Potamohiidcr , tribe Astacoicica, sub-order 

 Macrura, order Dccapocla, and sub-class Malanostraca. 



The possession of a thorax made up of eight segments and an 

 abdomen of six, places it among the Malacostraca : the presence of 

 a cephalothorax formed by coalescence of head and thorax, together 

 with that of movable eye-stalks, determines its position in the 

 division Eucarida : the modification of the first three pairs of 

 thoracic appendages as maxillipedes, and the arrangement of the 

 branchiae in three sets, place it in 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 Potamobiidee is distinguished by 

 having the podobranohise partly united to the epipodites, and by 

 possessing appendages 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 devia- 

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

 iateresting 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- 

 ag^es. As we have seen, the body consists of a prostomium, a 

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

 five metameres 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. 



There is considerable diversity of form among the BrancTiirypocl a. 

 Apus has already been described. Branchipus (Fig. 448, 1) and 



