In a subsequent moult the new limbs (maxillae, and anterior and
intermediate maxillipedes) come into action, and in this way the
Nauplius becomes a Zoea (Figure 29), agreeing perfectly with the Zoea of
the Crabs in the number of the appendages of the body, although very
different in form and mode of locomotion and even in many particulars of
internal structure. The chief organs of motion are still the two
anterior pairs of feet, which are slender and furnished with long setae;
the third pair of feet loses its branches, and becomes converted into
mandibles destitute of palpi. The labrum acquires a spine directed
forward and of considerable size, which occurs in all the Zoeae of
allied species. The biramose maxillipedes appear to assist but slightly
in locomotion. The forked tail reminds us rather of the forms occurring
in the lower Crustacea, especially the Copepoda, than of the spatuliform
caudal plate which characterises the Zoeae of Alpheus, Palaemon,
Hippolyte, and other Prawns, of the Hermit Crabs, the Tatuira and the
Porcellanae. The heart possesses only one pair of fissures, and has no
muscles traversing its interior like trabeculae, whilst in other Zoeae
two pairs of fissures and an interior apparatus of trabeculae are always
distinctly recognisable.

During this Zoeal period the paired eyes, the segments of the
middle-body and abdomen, the posterior maxillipedes, the lateral caudal
appendages and the stump-like rudiments of the feet of the middle-body
are formed (Figure 30). The caudal appendages sprout forth like other
limbs freely on the ventral surface, whilst in other Prawns, the
Porcellanae, etc., they are produced in the interior of the spatuliform
caudal plate.

As the feet of the middle-body come into action, simultaneously with