CRUSTACEA. 



413 



a 



morphologically to the eoxal joint of a limb, the following joints 

 developing into a palp-like appendage (mandibular /"'//>). Then 

 follow one or more pairs of weaker jaws (maxillce), and one or 

 more pairs of maxillipeds, which more or less resemble the legs 

 and, in parasitic forms, are often used for adhering (fig. 331). In 

 parasitic forms, the upper and under lips not unfrequently give rise 

 to a .suctorial proboscis, in which the styliform mandibles are placed. 

 The appendages of the thorax, of which at least three pairs are 

 present (Ostracoda), present an extremely various structure, in 

 accordance with the 

 mode of life and the 

 use made of them. 

 They are either broad 

 leaf - shaped swimming 

 feet (Phijllopoda], or bi- 

 ramous appendages 

 (Copepodci] ; they may 

 serve to produce currents 

 in the water like the 

 feet of the Cirripedia, 

 or they may be used for 

 crawling, walking, and 

 running (Isopoda, Deca- 

 poda}. In the latter 

 case, some of them end 

 with hooks or chelse. 

 Finally the appendages 

 of the abdomen, which 

 frequently itself moves 

 in toto and assists in 

 locomotion, are either 

 exclusively locornotory 

 as jumping or swim- 

 ming feet (Amphipoda), 

 in which case they usually differ from the appendages of the thorax ; 

 or they serve with their appendages for respiration, as well as for 

 carrying the eggs, and for copulation (Decapoda). 



The internal organization is not le.ss varied than is the external 

 form. 



In the lower forms, the nervous system often consists of a 

 ganglionic mass, which surrounds the oesophagus and is not further 



FIG. 331. Young stage (larva) <>f the Lobster (after G' 

 0. Sars). a, The lurva seen from tlie side ; -ff, ros- 

 trum ; A', A", antennae; Ef" third maxilliped; 

 /", anterior ambulatory leg. I, mandible with palp; 

 c. anterior maxilla with two blades and palp; ', pos- 

 terior maxilla with vibratile plate (scaphognathite) ; 

 e, first,/, second maxilliped. 



