1 64 BULLETIN OF THE BUREAU OF FISHERIES. 



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 ap£lied to the larva of a prawn and 

 craJi,Jn 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 arid ends in a forked telson. There is a long 

 depressed rostrum and a very long and sharp dorsal spine which 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 maxilUpeds and following thoracic legs for 

 swimming simply as a larva rather than as a zoea, however modified. 



Most true crabs and £rawns 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 Perusus and the schizopod Euphasia, hatch from eggs still more minute and 

 in a much simpler larval form called the nauplius. It is uiisegmented, possesses but 

 three pairs of appendages, representing the antennulae, 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 



fefifsrs or 



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 



