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 : 



(i) Nauplius and metanauplius. The shrimp PencBus 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 mysi^S, 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. brevicarpui), 

 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 ha.s 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 



