THE CLASSIFICATION OF CRUSTACEA 47 



scale- like exopodite, probably of use for keeping the 

 animal balanced in swimming. Only one pair of the 

 thoracic limbs are modified to form maxillipeds, and 

 all the legs (as in the larval Lobster) have exopodites 

 which form the chief swimming organs. The uro- 

 pods and telson form a " tail-fan." One of the most 

 curious points in the organization of some Mysidacea 



FIG. 16 Mysis relicta, ONE OF THE MYSIDACEA. ENLARGED. 

 (From Lankester's "Treatise on Zoology," after Sars.) 



cs, Cervical groove of the carapace ; m, brood-pouch 



is the possession of a pair of statocysts in the endo- 

 podites of the uropods. Each statocyst consists of a 

 small cavity containing a cake-shaped concretion 

 known as a " statolith," resting on a group of 

 sensory hairs. There is reason to believe that these 

 organs have the same function as the statocysts of 

 the Lobster, although they are placed at the other 

 end of the body. The statolith serves the same 

 purpose as the sand-grains found in the Lobster's 

 statocyst, although, unlike these, it is not introduced 



