38 THE APODID^R PART I 



those of the similar parts of other limbs (see Figs. 8 

 and 14, p. 59). Muscles also are found which are the 

 remains of the muscles which once ran into the now 

 completely degenerated dorsal parapodia (see section 

 on Musculature, p. 52). The redevelopment in 

 varying degrees of the dorsal parapodium in the 

 mandibles of some of the higher Crustacea, to 

 form the mandibular feelers, or palps (or perhaps 

 feeler- or palp-carriers) is a matter of considerable 

 interest. 



We see in these large fleshy mandibles of Apus an 

 undoubtedly primitive characteristic. They form a 

 perfect morphological transition between a limb like 

 the parapodium of the Annelida, and the hard special- 

 ised jaw of the Crustacea. The "teeth" are only 

 hard protuberances of the cuticle. The setae are very 

 small and grouped in tufts round the teeth. This 

 limb has no hinge on which it works ; it orobably 

 moves round its upper dorsal end as axis. 



The homologous limb in Limulus, i.e., the third, is 

 not so specialised as it is in Apus ; its masticatory 

 process is not more developed than that of the second 

 antennae, or of the two pairs of maxillae, and of the 

 first trunk limb. In Apus, only the mandibles work 

 between the upper and lower lips, but in Limulus all 

 the masticatory ridges of five pairs of limbs do so. 

 In Pterygotus (see Fig. 55, p. 239), the third limb 

 seems to have ceased to function as a jaw, and the 

 powerful swimming limb, the sixth or (morphologi- 

 cally) the first trunk limb, has developed strong masti- 

 catory processes, which seem to function as the chief 



