190 CRUSTACEA. 



II. Metamorphosis. 

 1. The Nauplius Stage. 



The most typical feature of the metamorphosis in the Crustacea, 

 and therefore, perhaps, the most convenient one with which to 

 commence our study, is the so-called Nauplius stage. In those cases 

 in which the young animal is hatched at a later stage of develop- 

 ment (e.g., in the Cladocera, the Arthrostraca, and most Decapoda), 

 the Nauplius stage is thrown back among the series of embryonic 

 stages; nevertheless, we commonly find that this stage is indicated 

 by a period of rest, such as an ecdysis of the Nauplius integument 

 (p. 118). 



The Nauplius body (Fig. 89) is as a rule oval, the anterior end 

 being the more rounded, and the posterior the narrower end of the 

 body. In other respects, there is great variety in the shape of 

 the body. Dorsally compressed, laterally compressed, long, and even 

 broad Nauplii are to be found. The possession of three pairs of 

 limbs the future first antennae (a'), second antennae (a"), and 

 mandibles (md) points to the fact that, in the Nauplius, we have 

 an already segmented larval form. This segmentation is, however, 

 not externally recognisable in the body of the Nauplius, although 

 the limits of the segments may be indicated in the ontogenetic 

 stages which lead up to the Nauplius (free-living Copepoda, Cirri- 

 pedia). Typically, the Nauplius has no shield-like reduplication of 

 the dorsal integument, but such a fold (which usually only appears 

 at a later stage) may be distinctly recognisable in individual cases 

 (Cirripedia, Fig. 102 A), or at least may be indicated by a slight 

 fold in the integument of the dorsal surface. The posterior end 

 of the body is still without the paired furcal processes, but is armed 

 with paired setae (furcal setae). The most anterior pair of limbs, 

 the first antennae (a'), are uniramose, and consist of few joints or 

 are still unjointed. They serve for locomotion, and are also of 

 importance as bearing sensory organs. The two pairs of limbs 

 which follow these are developed in the form of biramose swim- 

 ming limbs. The first of these, the second antennae (a"), lie at 

 the sides of the mouth, and are distinguished by a strong, hook-like, 

 masticatory process springing from the basal joint. The third pair 

 of limbs, the mandibles (md), also function principally as locomotory 

 organs. A masticatory blade is not, as a rule, developed on the 

 basal joint, indeed, this structure, so characteristic of the appendage 

 in the adult, is hardly indicated, although it may in some cases 



