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GENERAL NOTES ON CRUSTACEANS. 315 
Nauplius becomes the head region of the adult; the 
posterior region also persists; the new growth of segments. 
and appendages takes place (with numerous moultings) in 
the region between these. 
The second important form of larva is the Zozea, 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 Zozea stage. 
(a) The crayfish (Astacus) is hatched almost as a miniature adult. 
The development is therefore very direct in this case, 
(4) The lobster (Homarus) is hatched in a AZysds 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 Zoga form, and pass with moults through 
a Megalopa stage, with the abdomen in a line with the cephalo- 
thorax. The abdomen is stibsequently tucked in under the 
thorax. : 
(d) Penaeus (a kind of shrimp) is hatched as a Mazplius, becomes a 
Zoea, then a Myszs, then an adult. Its relative Luczfer starts 
as a Meta-Nauplius with rudiments of three more appendages 
than the Nauplius. Another related form, Sevgestes, is hatched 
as a Protozoea, with a cephalothoracic shield and an unseg- 
mentedabdomen. Thus there are two grades between Nauplius. 
and Zoza. 
Three facts must be borne in mind in thinking over the life histories. 
of crayfish, lobster, crab, and Pexeus: (1) 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 larve , 
exhibit characters which are related to their own life rather than to that 
of the adult ; (3) it is a gezera/ 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 Zoea stage 
is antecedent to the A/yszs, etc.,—yet it does not follow that ancestral 
Crustaceans were like Nauplii. On the contrary, the Vauzplcus must be 
regarded as a larval reversion to a type much simpler than the ancestral 
Crustacean. 
C&cology.—Most Crustaceans are carnivorous and pred- 
atory; others feed on dead creatures and organic débris in 
the water ; a.§4inOrifgdepend 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 
