10 THE LIFE OF CRUSTACEA 



together. This is shown by a comparison with 

 certain shrimp-Hke Crustacea (Mysidacea) in which 

 the carapace arises, Hke a fold of the skin, from the 

 hinder edge of the head, and envelops, like a loose 

 jacket, the distinctly segmented thorax. In the 

 Lobster this fold has become adherent to the 

 thoracic somites down the middle of the back, but 

 at the sides it hangs free, enclosing on each side 

 a cavity within which lie the gills. 



It seems at first sight 'strange to include in the 

 same category as "limbs" or " appendages " organs 

 which differ so much in form and function as do 

 the swimmerets, the walking legs, the jaws, and the 

 antennae. Nevertheless it can easily be demon- 

 strated that all of them are constructed on the same 

 general plan, and arise in the embryo from rudiments 

 which are, for the most part, exactly alike. This is 

 expressed in technical language by saying that the 

 appendages of the whole series are homologous with 

 one another. A full discussion of this interesting 

 fact would require more space than can be devoted 

 to it here, but a few examples may be given to illus- 

 trate what is meant by the " serial homology " of 

 the appendages in Crustacea. 



- If one of the swimmerets be detached from the 

 third abdominal somite, it will be seen (Fig. 2) to 

 consist of a stalk, known as the proiopodite, bearing 

 two branches, of which that on the outer side is 

 called the exopodite, and that on the inner side the 



