SECT. XII RELATION OF APUS TO CRUSTACEA 191 



also develop powerful seizing hooks. The homology 

 of the second pair of limbs of Apus with the second 

 antennae of the Apodida^ is doubly interesting because 

 we here find the ventral parapodium retained as 

 masticatory ridge. The great number of masticatory 

 ridges in Limulus will be referred to again, and com- 

 pared with the number of homologous ridges in Apus 

 and the fossil Crustacea. 



The third pair of limbs of Limulus offers a most 

 interesting comparison with the mandibles of Apus. 

 In the latter, the ventral parapodium alone is retained, 

 the dorsal having entirely disappeared. In Limulus, 

 both have been retained, the ventral parapodium as a 

 very pronounced masticatory ridge, the dorsal as a 

 long jointed chelate leg. 



The fourth and fifth head limbs have nearly the same 

 form as the third, and are homologous with the two 

 pairs of maxillae of Apus. One interesting feature, 

 however, deserves special attention in connection 

 with our deduction of Apus and also of Limulus 

 from an Annelid. In Limulus, the ventral para- 

 podium, which stands out much more pronounced 

 as a ventral parapodium than it does in Apus, has 

 retained distinct traces of its sensory cirrus (see Fig. 

 44). In this respect Limulus is more primitive than 

 Apus. On the other hand in Limulus, the sensory 

 cirrus (or exopodite) disappears from the dorsal 

 parapodium of the head limbs ; this is the exact 

 opposite of what we find in the typical Crustacean 

 limb, where the dorsal parapodium as endopodite and 

 its sensory cirrus as exopodite are alone preserved, the 



