t 
NATURAL HISTORY OF AMERICAN LOBSTER. 1 65 
a shorter pelagic period. The size of the individual egg is increased, but the number 
of eggs is diminished. The alternative lies between two extremes as follows: 
( Eggs small, but many of them. 
Long metamorphosis. 
Less chance for individual survival, but more individual chances. 
( Eggs large, but few in number. 
Metamorphosis shortened. 
Greater chance for the individual, but fewer individuals to take it. 
Between these two types of adjustment many compromises have been made. The 
principal larval stages or types in decapods which have received definite names, being 
the survivals in some cases of a period when crustacean larvae were considered adult 
forms, are the following: 
(1) Nauplius and metanauplius. The shrimp Penceus is hatched as a nauplius and 
passes through the metanauplius, first and second protozoea, first and second zoea, and 
mysis stages, before attaining the adult form. Lucifer hatches as a nauplius, molts 
into a metanauplius stage, with buds of three more appendages present ; then passes 
successively through the protozoea, zoea, schizopod or mysis, and mastigopus stages, 
and finally to the adult. 
(2) Protozoea, zoea, and metazoea. The shrimps Sergestes and Stenopus hatch as 
protozoeas, and pass the successive stages as given for Lucifer. 
In the protozoeas the antennae are large and are often used in swimming; the 
carapace is formed, and the abdomen is unsegmented or but incompletely marked off 
into somites. The telson is forked and garnished with plumose setae. 
A protozoean stage has been assigned to the lobster, but erroneously, as will be later 
explained. 
The zoea characteristic of the crabs has seven pairs of appendages and a segmented 
abdomen. The last two pairs — first and second maxillipeds ( Callinectes ) — are swimming 
feet, which in the adult are converted into mouth parts. Many shrimp are hatched as 
modified zoeas with three pairs of locomotor maxillipeds, and the abbreviation is carried 
a step farther in some species of Synalpheus (S. minus) where buds of three pairs of 
thoracic limbs appear behind the maxillipeds, and still farther in others (S. bremcarpus), 
where the first young to appear are in a “mysis” stage similar to the second larva of 
the lobster. 
(3) Megalopa. The changes which follow in the early development in the crab 
zoea lead first to the metazoea, with rudimentary thoracic limbs and pleopods, and 
then by a sudden leap to the megalopa, a form comparable to the fourth stage of the 
lobster. The megalopa has large, free, stalked eyes, large claws, and functional walking 
legs. The swimming exopodites or outer branches of the maxillipeds have atrophied 
and disappeared, and like a lobster from the fourth stage onward, it has a segmented 
abdomen with functional swimmerets. It has also well-developed statocysts or balanc- 
ing organs and no longer reels in its motion through the water by day, but maintains a 
definite, upright position. In the course of succeeding molts the abdomen becomes 
reduced and modified, while the animal acquires the peculiar structure and habits of 
