346 



CRUSTACEA. 



occurred in many groups, resulting in their loss, in varying 

 degrees, of a locomotory function, and their adaptation to 

 subserve the transference of food to the mouth. The thoracic 

 limbs thus modified are known as maxillipeds. 



The appendages of the thorax vary greatly in number and 

 shape. In the Polycopidae (Ostracoda) they are absent alto- 

 gether, locomotion being effected entirely by the limbs of the 

 head ; in other Ostracoda there are two pairs, in the Cladocera 

 4-6, and in the Phyllopoda 11-19 pairs. In the Malacostraca 

 the number is almost constantly (except in obviously degraded 



forms) eight. In the 

 Apodidae there are 

 40-63 pairs of post- 

 cephalic appendages, 

 presenting a gradual 

 transition in form 

 and size ; but the 

 presence of the open- 

 ing of the genital duct 

 on the segment bear- 

 ing the llth permits 

 us to regard the eleven 

 anterior as thoracic. 



The modifications 

 which befall the epi- 

 podites of the thoracic 

 limbs present an in- 

 teresting series. They 

 are generally branchial 

 in character and may 

 be simple lobed plates 

 (Phyllopoda, Anas - 

 pides(Fig. 238), Am- 

 phipoda) or variously 

 subdivided (Schizopoda, Decapoda) ; but in certain groups of 

 Malacostraca (" Peracarida," cf. p. 455) the epipodites of 

 certain of the legs appear to have become modified in the female 

 as oostegites, to form, together with their fellows, a brood pouch 

 beneath the thorax, in which the young are protected. 



Except in some of the Branchiopoda (as A pus, to which 



FIG. 240. Larva of Lobster (after G. O. Sars). a ttfe 

 larva seen from the side ; A', A" first and second 

 antennae ; I* the chelate first ambulatory legs (4th 

 thoracic). Like the other thoracic limbs they carry 

 exopodites in the larva. K f" third maxilliped ; b 

 mandible, with palp ; c anterior or first maxilla ; d 

 posterior or second maxilla (with scaphognathite) ; e 

 and / first and second maxillipeds. 



