BULLETIN OF THE BUREAU OF FISHERIES. 
164 
Few general laws are without exceptions, and the fact that metamorphosis, which 
is even more common in Crustacea than in insects, is sometimes scamped or wanting 
altogether, led at once to confused and contradictory ideas. The abbreviated larval 
history of the crayfish which had been worked out with great care by Rathke in 1829 
and that of the European lobster first announced by Thompson ( 262 ) in 1831, and 
confirmed by Brightwell in 1835, as well as that of the West Indian shore crab, 
Gegarcinus ruricola, determined at the same time by Westwood, led to temporary 
difficulties, which were eventually cleared away when the development of many kinds 
of both macruran and brachyuran Crustacea had been studied with sufficient care. 
It thus appears that the term “zoea” was first applied to the larva of a prawn and 
crab, in which the swimming appendages are three pairs of claw feet or maxillipeds, 
the thoracic legs being rudimentary buds when represented at all. The abdomen is 
segmented, but bears no appendages and ends in a forked telson. There is a long 
depressed rostrum and a very long and sharp dorsal spine ivhich springs from the 
middle of the carapace, both of which seem to be admirably adapted for protection. 
Though many variations occur in the larvae of closely related genera and it is difficult 
to make general terms fit the varying degrees of modification which larvae have under- 
gone, it seems best to preserve the historical usage of the word zoea as far as possible. 
For this reason we speak of the young lobster when hatched with its thoracic appen- 
dages well formed and using both its great maxillipeds and following thoracic legs for 
swimming simply as a larva rather than as a zoea, however modified. 
Most true crabs and prawns hatch as zoeas from minute eggs, and are commonly 
translucent and flecked with brilliant red and yellow pigment cells. They molt fre- 
quently during the first few weeks of life, passing in the case of crabs through a megalops 
stage, and then gradually assuming the structure and habits of the adult animal. 
Entomostraca generally, and exceptionally certain of the Malacostraca, such as the 
decapod Penceus and the schizopod Euphasia, hatch from eggs still more minute and 
in a much simpler larval form called the nauplius. It is unsegmented, possesses but 
three pairs of appendages, representing the antennulas, antennae, and mandibles of the 
adult, and has a single median “nauplius” or “Cyclopean” eye. Upon the theory 
of recapitulation, the nauplius has been regarded as the representative of a primitive 
or ancestral form, but it seems more probable that existing larvae of this type have 
become modified to meet the present conditions of their environment. 
In every metamorphosis individuality is preserved from egg to adult, and develop- 
ment proceeds according to this simple formula : Egg = embryo = larva 1,2,3+= adolescent 
( gcrcrs or 
sperm 
A long metamorphosis which entails a long pelagic life near the sunace means greater 
risk and greater destruction than one of short duration. Consequently it is not surpris- 
ing to find a general tendency to shorten this larval period, reducing the metamorphosis 
by shifting it to the egg, or, more exactly, by lengthening the period of egg development. 
In this case the supply of food yolk is increased to support a longer life within the egg 
membranes, and the larvae or young issue in a more advanced state, and as a rule have 
