NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 55 



completed for better security in the egg. The Hfe history of Pcnaeus 

 setiferiis (L.) is now well known from the studies of Pearson (1939) 

 and Heegaard (1953), and will here be briefly reviewed from the 

 papers by these two authors. The penaeid life history, moreover, will 

 serve also as a good subject for a discussion of the significance of 

 crustacean larval forms. 



Both the nauplius and the metanauplius of Penaeus (fig. 21 A, B) 

 have long swimming appendages, but the alimentary canal is not yet 

 developed and the larvae in these stages subsist on the yolk derived 

 from the egg. In the metanauplius (B), however, the mandibles have 

 acquired gnathal lobes on their bases, and rudiments of four pairs of 

 postmandibular appendages are present, the last being those of the 

 second maxillipeds {2Mxpd). The metanauplius goes over into the 

 zoeal stage, in which there are three instars. In the first zoea (C) 

 the carapace has developed, the mandibles have become functional 

 jaws, and the larva now takes its first external food. The following 

 appendages have developed into biramous limbs, and the abdomen 

 shows a faint trace of segmentation, but the larva apparently still 

 swims by means of the antennae. In the third zoeal instar (D) the 

 larva takes on something of the adult form (G). The carapace 

 covers the thorax, and rudiments of the pereiopods (D, Prpds) are 

 present, the abdomen is fully segmented but pleopods have not yet 

 appeared, and the antennae are still the chief organs of propulsion. 

 The third zoea is follow-ed by the so-called mysis stage, which goes 

 through two instars. In the first mysis (E) the pereiopods are all 

 present and have long seta-bearing exopodites, which now assume 

 the locomotor function, and the antennae are reduced. The abdomen 

 has well-developed uropods, but pleopods are as yet absent. 



The next stage (fig. 21 F), known as the postinysis, or postlarra, 

 more nearly resembles the adult. The pereiopods have lost their 

 exopodites, and those of the first three pairs are chelate. Slender 

 uniramous pleopods are present on the abdomen and are now the 

 swimming organs as they are in the adult. In the adult (G) the 

 pleopods have acquired the typical biramous structure, and a long 

 filamentous flagellum arises from each second antenna. 



The life-history stages of Lucifer, as described by Brooks (1882), 

 are similar to those of Penaeus, except that the larva hatches as a 

 metanauplius and the animal takes on a different form in its preadult 

 and mature stages. Numerous examples of the bizarre larvae of 

 Sergestidae, characterized by long, branched spines on the thorax and 

 abdomen, are illustrated by Gurney (1924). 



There has been much discussion among carcinologists as to whether 

 or not the forms of decapod larvae have a phylogenetic significance. 



