A, Nauplius; B, young zoëa; C, older zoëa; D, early "schizopod" stage

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In the further development of the larva, the body increases in length and becomes divided into somites which increase in number by new somites appearing behind those already marked off; the rudiments of the limbs also appear in regular order from before backwards; the dorsal shield of the nauplius grows out into a carapace, beneath which the paired eyes begin to develop in front. Thus after passing through metanauplius and protozoëa stages ([Fig. 29], B) the larva becomes a zoëa ([Fig. 29], C), resembling that of the Crab already described in that the swimming organs are the maxillipeds, but differing in having the uropods well developed and forming a tail-fan at the end of the abdomen, the hinder thoracic somites marked off and their appendages present as rudiments, and the stalked eyes free from the carapace. This is followed by a schizopod stage ([Fig. 29], D), in which the prawn-like shape is assumed and the thoracic legs have large exopodites used for swimming. Later these exopodites diminish in size, though they do not quite disappear in the adult Penæus, and the function of swimming organs is taken over by the abdominal swimmerets.

In Penæus the larvæ are of comparatively simple form, but in the allied genus Sergestes the zoëa has a very remarkable appearance. The carapace is armed with long spines, each bearing two comb-like rows of secondary spines. The development of spines and other outgrowths of the surface of the body is a very common characteristic of organisms that, like these larvæ, float or swim in the open sea; its probable significance will be discussed in a later chapter.

The shrimp-like Euphausiacea have a larval development very like that of Penæus. Most, if not all, of the species are hatched from the egg in the nauplius stage, and pass through stages very similar to those described above. The adult animals, however, may be said to remain in the "schizopod" stage, since the exopodites of the thoracic legs remain large and are used in swimming.

Fig. 30—Newly-hatched Young of a Crayfish (Astacus fluviatilis). Enlarged

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