228 ARTHROPODS. 



takes place (with numerous moultings) in the region between 



these 



The second important form of larva is the 2^<««, which 

 has all the appendages on to the last maxillipedes inclusive, 

 an unsegmented abdomen, and two lateral eyes m addition 

 to the unpaired one of the Nauplius stage. As such most 

 Decapoda are hatched. . . 



(a) The crayfish {Astacus) is hatched almost as a miniature 

 adult. The development is therefore very direct in 

 this case. 

 lb) The lobster {Homarus) is hatched in a Mysts stage, 

 in which the thoracic limbs are two-branched and 

 used for swimming. After some moults it acquires 

 adult characters. 

 {c) Crabs are hatched in the Zocea form, and pass with 

 moults through a Megalopa stage, in which they 

 resemble certain Hermit-Crabs. The abdomen is 

 subsequently tucked in under the carapace. 

 (d) PencBus (a kind of shrimp) is hatched as a Nauplius, 

 becomes a Zoma, then a Mysis, then an adult. Its 

 relative Lucifer starts as a Meta-Nauplius with rudi- 

 ments of three more appendages than the Nauplius. 

 Another related form, Sergestes, is hatched as a 

 Protozoaa, with a cephalothoracic shield and an 

 unsegmented abdomen. Thus there are two grades 

 between Nauplius and Zosea. 

 Three facts must be borne in mind in thinking over the 

 life-histories of crayfish, lobster, crab, zxi^Penaus : — (i) there 

 is a general tendency to abbreviate development, and this is 

 of more importance when metamorphosis is expensive and 

 full of risks ; (2) there is no doubt that larvae exhibit char- 

 acters which are related to their own life rather than to that of 

 the adult; (3) it is a general truth, that in its individual develop- 

 ment the organism has to recapitulate to some extent the 

 evolution of the race, that ontogeny recapitulates phylogeny. 

 But while there can be no doubt that the metamorphoses of 

 these Crustaceans is to some extent interpretable as a recapit- 

 ulation of the racial history, for there were unsegmented 

 animals before segmented forms arose, and the 2^ma stage 

 is antecedent to the Mysis, etc., yet it does not follow that 

 Crustaceans were once all like Nauplii. Moreover, this idea 



