27S CRUSTACEA. 



Compared with Astacus, however, the most important 

 point we have to notice is the frequent occurrence of a very 

 striking metamorphosis in the life history. In other words, 

 the larva hatched from the egg is rarely like the parent, and 

 only acquires the adult characters after a series of profound 

 changes. In some cases [Nebalia, Mysis) a metamorphosis 

 takes place within the egg-cases, and in the few forms in 

 which development seems to be direct, slight traces of meta- 

 morphosis are found. 



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 (as 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. 

 (/>) The lobster (ffomarus) 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. 

 (<■) Crabs are hatched in the Zoaa form, and pass with moults through 



a Megahpa stage, in which they resemble certain Hermit crabs. 



The abdomen is subsequently tucked in under the thorax. 

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



Zoaa, 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 fmm, Sergestes, is hatched 



