494 



CRUSTACEA. 



structure in different groups of the Crustaceans. In the Decapod 

 Crustaceans five pairs of the thoracic appendages are in the form 

 of walking-legs ; while other groups (such as the Isopods) have 



seven pairs of ambulatory limbs ; 

 and in others, again, the number of 

 locomotive thoracic limbs may be 

 greater or less than the above. The 

 abdominal appendages are very vari- 

 able in form and function, but they 

 are mostly primarily concerned in 

 locomotion, though often modified 

 to serve as respiratory organs, or, in 

 the females, carrying the eggs. 



As regards their classification, the 

 Crustacea may be divided into the 

 two primary sections of the Entomo- 

 straca and the Malacostraca. The 

 first of these includes a great num- 

 ber of comparatively simple Crusta- 

 ceans, in which the number of the 

 segments and appendages is very 

 variable, sometimes rising above the 

 below it. The second division in- 

 cludes the more highly organised Crustaceans, in which the num- 

 ber of the segments and appendages is definite. The Cirripedes 

 (with the Rhizocephala) may be provisionally regarded as a third 

 division under the name of Anchor acephala, characterised by the 

 fact that the adult is attached to foreign bodies by the metamor- 

 phosed head. The orders included in these three divisions are 

 shown in the following table : — 



Fig. 353. — The third abdominal somite 

 of the Lobster, separated, t, Tergum ; jt, 

 Sternum; /, Pleura; a, Protopoditeof the 

 appendage; b, Exopodite ; c, Endopodite. 



normal, and sometimes fallim 



Sub-class I. Anchoracephala. 

 Order 1. Cirripedia. 



Sub-class II. 



Order 1. Ostracoda. 

 Copepoda. 

 Cladocera. 

 Phyllopoda. 

 Phyllocarida. 



6. Tr Hob it a. 



7. Xiphosura. 



8. Eurypterida. 



Entomostraca. 



> Legion , Lophyropoda. 



■Legion, Branchiopoda. 



Legion, Merostomata. 



