282 CRUSTACEA. 



Almost all the lower Crustaceans and the higher forms 

 Euphausia and Penceus are hatched in a Nauplius stage. 

 In the remaining cases the Nauplius stage is indicated 

 within the egg by the moulting of a larval cuticle (so in 

 Astacus}. The Nauplius is characterised by a typically 

 rounded body, and by the presence of three pairs of append- 

 ages, which are the only obvious indications of segmenta- 

 tion. The first pair of appendages are unbranched and 

 bear larval sense organs, the next two are biramose swim- 

 ming organs. There is an unpaired median eye, but no 

 heart and frequently no hind gut. The three pairs of 

 appendages become the first and second pairs of antennae 

 and the mandibles of the adult. The head region of the 

 Nauplius becomes the head region of the adult, the posterior 

 region also persists, the new growth of segments and append- 

 ages takes place (with numerous moultings) in the region 

 between these. 



The second important form of larva is the Zoaea, which 

 has all the appendages on to the last maxillipedes inclusive, 

 an unsegmented abdomen, and two lateral compound eyes 

 in addition to the unpaired one of the Nauplius stage. 

 Most Decapoda are hatched in the Zoaea stage. 



- (a) The crayfish (Astacus] is hatched almost as a miniature adult. 

 The development is therefore very direct in this case. 



(b) The lobster (Homarus} is hatched in a Mysis 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 thorax. 



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



Zocea, then a Mysis, then an adult. Its relative Lucifer starts 

 as a Meta-Nauplius with rudiments of three more appendages 

 than the Nauplius. Another related form, Sergestes, is hatched 

 as a Protozocea> with a cephalothoracic shield and an unseg- 

 mented abdomen. Thus there are two grades between Nauplius 

 and Zocea. 



Three facts must be borne in mind in thinking over the life histories 

 of crayfish, lobster, crab, and Penceus: (l) there is a general tendency 

 to abbreviate development, and this is of more importance when meta- 

 morphosis is expensive and full of risks ; (2) there is no doubt that larvae 

 exhibit characters which are related to their own life rather than to that 

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

 the organism has to recapitulate to some extent the evolution of the 

 race, that ontogeny recapitulates phylogeny. But while there can be no 



