300 PHYL UM A R THR OPODA . 



segments and appendages 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, 

 a segmented abdomen, and two lateral compound eyes, 

 in addition to the unpaired one of the Nauplius stage. 

 Most Decapoda are hatched in the Zosea stage. 



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



The development is therefore very direct in this case. 



(b] The lobster (Honianis} is hatched in a My sis 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 Zoeea 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] Pemzus (a kind of shrimp) is hatched as a Nauplius, becomes a 

 Zocea, then a My sis, then an adult. Its relative Lucifer starts 

 as a Meta-Nanplius 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 Zoaea. 



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

 of crayfish, lobster, crab, and Pemeus : (i) There is a general tendency 

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

 morphosis is expensive and full of risks 5(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 recapitulates to some extent the evolution of the race, that 

 ontogeny tends to recapitulate phylogeny. But while there can be no 

 doubt that the metamorphosis of these Crustaceans is to some extent 

 interpretable as a recapitulation of the racial history, for there were 

 unsegmented animals before segmented forms arose, and the Zocca stage 

 is antecedent to the My sis, etc., yet it does not follow that ancestral 

 Crustaceans were like Nauplii. On the contrary, the Nauplius must be 

 regarded as a larval reversion to a type much simpler than the ancestral 

 Crustacean. 



Bionomics. Most Crustaceans are carnivorous and pre- 

 datory ; others feed on dead creatures and organic debris 

 in the water ; a minority depend upon plants. Many of the 

 smaller forms play a very important part in the economy of 

 nature in the circulation of matter for while they feed on 

 animalcules and debris they are themselves the food of 

 larger animals such as fishes. 



Parasitism occurs in over 700 species, in various degrees, 

 and, of course, with varied results. Most of the parasites 



