118 CKUSTACEA. 



pound ; antennae generally four ; jaws generally 

 three pairs ; foot-jaws (to the same number), of 

 which the outer pairs often have the office of feet ; 

 and feet (usually five pairs, or, when the foot-jaws 

 become feet in function, seven pairs). The skin is 

 usually a solid crust, more or less calcareous, 

 which is cast off at intervals during growth. In 

 the early moultings they sustain a true meta- 

 morphosis. 



Mouth prolonged into a sucker . . . Thelastia. 

 Mouth armed with jaws Dactia. 



Section I. Thelastia. 



Feet formed for walking, and well- 

 developed ; mouth without distinct 

 jaws Podosomata. 



Feet formed for swimming, or 

 rudimentary : mouth armed with 

 style-like jaws : 



Thorax unjointed; antennae de- 

 prived of appendages ; feet always 

 rudimentary or deformed ; foot-jaws 

 rudimentary Epizoa. 



Thorax with many distinct joints, 

 carrying three or four pairs of feet ; 

 foot-jaws well- developed .... SipJwnostomata. 



Order L PODOSOMATA. 



Head lengthened, with a three -lobed mouth. 

 Thorax consisting of four segments ; with a rudi- 

 mentary tubular joint, representing the abdomen. 

 Head unfurnished with appendages. Eyes four, 

 grouped on a wart, on the first joint of the 

 thorax. Both sexes have four pairs of walking 



