106 



ANIMAL BIOLOGY 



Endopodite 



Protopodite"' 



Protopodite 



A. 1 



Exopodite 



Endopodite 



Mx.l 



Exopodite 

 Endopodite 



M. 



..Endopodite 

 Endopodite 



Mp. 1 



Epipodite 



Endopodite 

 Protopodite... 



Mp. 3 



Endopodite. 



Protopodite 



Epipodite 



.Exopodite 



Mx. 2 



•Epipodite 



Mp. 2 



Exopodite 



Exopodite 



Endopodite. 

 Protopodite 



Epipodite 



Chitinous threads 



The body of a primitive Ar- 

 thropod differs from that of the 

 Earthworm chiefly in the reduc- 

 tion of the number of segments 

 and the development of paired 

 jointed appendages as outgrowths 

 from the body in each segment. 

 From such a primitive type all 

 the multitude of diverse forms of 

 Arthropod bodies can be derived. 

 For instance, in the Crayfish, 

 which is essentially a fresh-water 

 Lobster, the body consists of 

 twenty-one segments, of which 

 segments 1 to 6 together form 

 the head; segments 7 to 14, the 

 thorax; and segments 15 to 21, 

 the abdomen. In other words, 

 by the union or complete fusion 

 of certain segments, the body has 

 become divided into more or less 

 distinct regions. (Figs. 62, 63.) 



Furthermore, the primitive lo- 

 comotor appendages of the re- 

 spective segments have become 

 modified into organs for widely 

 different functions: those of the 

 head, as sensory organs, jaws, 

 etc. ; those of the thorax, as organs 

 for grasping, offense and defense, 

 and walking; and those of the 

 abdomen for swimming, etc. Thus 

 change in structure has gone on 



Fig. 64. — Typical appendages of a 

 Crayfish. All have been derived from a 

 simple biramous appendage. Protopo- 

 dite, endopodite, and exopodite are ho- 

 mologous throughout the series. A. 1, 

 antennule; A2, antenna; L. 4, fourth walking leg; M., mandible; Mp. 1, first 

 maxilliped; Mp. 2, second maxilliped; Mp. 3, third maxilliped; Mx. 1, first 

 maxilla; Mx. 2, second maxilla. (From Hegner, after Kerr.) 



