54 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



segmented and the uropods are developed. In the furcillia stage 

 (E, F) the larva begins to resemble the adult. The eyes are now 

 Stalked and project from beneath the carapace. The first furcillia 

 instar (E) has still only the appendages of the metanauplius, but after 

 the first moult the pereiopods appear as simple papillae, which later 

 enlarge (F, G) and finally become biramous appendages. At the same 

 time the pleopods are formed. According to Lebour (1925) in 

 Nyctiphanes and Meganyctiphanes there are 12 furcillia instars sepa- 

 rated by moults. In the final cyrtopia stage (G), after 8 to 13 moults 

 according to the species, the young euphausiid acquires the adult 

 structure with a complete set of appendages and luminescent organs. 



DECAPODA 



The decapod crustaceans include the shrimps, lobsters, crayfishes, 

 and crabs. None of them exhibits any pronounced metamorphic 

 changes during development or in the adult stage, but most of them 

 go through stages of growth characterized chiefly by the successive 

 development of sets of appendages. Only in the Penaeidae is there 

 a free naupHus and a metanauplius. Most species hatch in a form 

 . called a zoea, in which the appendages following the second maxilli- 

 ,^j)eds are as yet undeveloped or are present as rudiments. With the 

 functional completion of the pereiopods the larva is l<nown as a my sis 

 from its fancied resemblance to a member of the Mysidacea. Some 

 species, however, go through the zoea stage in the egg and hatch as 

 a mysis, and a few are almost completely developed in the adult form 

 on leaving the tgg. 



The decapod larvae are free swimming, and in general are fairly 

 uniform in structure with a fully developed carapace and a long seg- 

 mented abdomen. A few, however, take on unusual forms. Among 

 the Sergestidae many of the larvae are characterized by a great de- 

 velopment of long, often profusely branched spines on the thorax and 

 abdomen. The rounded carapace of the palinuran Polycheles larva 

 looks like a spiny burr, and others of the same group, known as 

 phyllosome larvae, are broad, flat, and leaflike in shape. Presumably 

 such forms are adaptations to buoyancy or floating. 



The Penaeidea. — In this order the family Penaeidae is of particular 

 interest because it includes the only decapods that begin life as free- 

 swimming nauplii. The fact that among the Malacostraca both the 

 penaeids and the euphausiids hatch from the egg as nauplii may be 

 taken as evidence that primarily all the crustaceans hatched at this 

 early stage of development, and that later hatching among the higher 

 Malacostraca is secondary, resulting from the earlier stages being 



