SECT. XII RELATION OF APUS TO CRUSTACEA 207 



The spermatozoa are filiform as in the carnivorous 

 AnneHds, but this fact is of no great morphological 

 importance. The genital aperture is situated on the 

 posterior face of the operculum, i.e. on the second 

 trunk limb ; in Apus it is between the tenth and 

 eleventh trunk feet. There were originally nephridial 

 openings between the limbs of all the more developed 

 trunk segments ; hence this difference between Limulus 

 and Apus is of no importance. 



Development. — We have already pointed out that 

 the absence of the Nauplius stage in Limulus is no real 

 difficulty. We should only expect a Nauplius stage 

 in Limulus inasmuch as the Nauplius is the larva of 

 the original Crustacean-Annelid. The great speciali- 

 sation of Limulus, apparently so unlike its Annelidan 

 ancestor, readily explains its direct development 

 without passing through any such stage. Its meta- 

 morphoses are all passed through within the (tg^ ; 

 we thus learn nothing of its early ancestors. Its 

 so-called " Trilobite stage " receives, however, a new 

 interest from our theory, which includes the Trilobite 

 also among the descendants of the same bent Annelid. 



We conclude, then, from the comparison between 

 Apus and Limulus that both animals have developed 

 from the same bent Crustacean-Annelid ; hence the 

 similarity in their organisation. Although their further 

 development has travelled along slightly different 

 lines, their striking differences are in most cases easily 

 explained by the one having retained more primitive 

 Annelidan characteristics than the other. 



