The development begins with a free nauplius stage.
- In this division the body is partly covered by a broad shield, united in front with the head; the eyes are sessile, the first antennae are small, the second rudimentary or wanting; of the numerous feet, sometimes sixty-three pairs, exceeding the number of segments to which they are attached, the first pair are more or less unlike the rest, and in the female the eleventh have the epipod and exopod (flabellum and sub-apical lobe of Lankester) modified to form an ovisac. Development begins with a nauplius stage.
The development usually begins with a nauplius stage (Sars, 1896, 1900).
Only the embryos of Leptodora are known to hatch out in the nauplius stage.
By the god Amymone became the mother of Nauplius, the wrecker.
The three pairs of appendages present in the " nauplius " larva show certain peculiarities of structure and development which seem to place them in a different category from the other limbs, and there is some ground for regarding the three corresponding somites as constituting a " primary cephalon."
Female; b, first larval stage (Nauplius); c, second larval 3, Lepidurus Angassi: a, dorsal stage.
Aspect; b, ventral aspect of 6, Nauplius of Artemia salina.
They are absent in the earliest and most primitive B larval forms (nauplius), and appear only late in the course of development, after many of the trunk-limbs are fully formed.
The antennules (or first antennae) are almost universally regarded as true appendages, though they differ from all the other appendages in the fact that they are always innervated from the " brain " (or preoral ganglia), and that they are uniramous in the nauplius larva and in all the Entomostracan orders.
In the nauplius larva they lie rather at the sides than in front of the mouth, and their basal portion carries a hook-like masticatory process which assists the similar processes of the mandibles in seizing food.
The mandibles, like the antennae, have, in the nauplius, the form of biramous swimming limbs, with a masticatory process originating from the proximal part of the protopodite.
The eyes of Crustacea are of two kinds, the unpaired, median or "nauplius " eye, and the paired compound eyes.
The former is generally present in the earliest larval stages (nauplius), and in some Entomostraca (e.g.
Further, a definite cuticular membrane is frequently formed and shed at this stage, which corresponds to the nauplius-stage of larval development.
The typical nauplius (fig.
- Nauplius of a Prawn (Penaeus).
A nauplius larva differing only in details from the typical form just described is found in the majority of the Phyllopoda, Copepoda and Cirripedia, and in a more modified form, in some Ostracoda.
Among the Malacostraca the nauplius is less commonly found, but it occurs in the Euphausiidae among the Schizopoda and in a few of the more primitive Decapoda (Penaeidea) (fig.
It seems certain, therefore, that the possession of a nauplius larva must be regarded as a very primitive character of the Crustacean stock.
As development proceeds, the body of the nauplius elongates, and indications of segmentation begin to appear in its posterior part.
The course of development here outlined, in which the nauplius gradually passes into the adult form by the successive addition of somites and appendages in regular order, agrees so well with the process observed in the development of the typical Annelida that we must regard it as being the most primitive method.
In the Cirripedia, for example, the latest nauplius stage (fig.
13, B), differing widely from the nauplius in form, and possessing all the appendages of the adult.
Besides the nauplius and the zoea there are many other types of Crustacean larvae, distinguished by special names, though, as their occurrence is restricted within the limits of the smaller systematic groups, they are of less general interest.
This is well seen in the nauplius of many Cirripedia (fig.
The various larval forms, especially the nauplius and zoea, were supposed to reproduce, more or less closely, the actual structure of ancestral types.
As regards the nauplius, however, the constancy of its general character in the most widely diverse groups of Crustacea strongly suggests that it is a very ancient type, and the view has been advocated that the Crustacea must have arisen from an unsegmented nauplius-like ancestor.
- Nauplius 'of Tetraclita porosa after the first moult.
In those Copepods in which the palps of the mandibles as well as the antennae are biramous and natatory, the first three pairs of appendages retain throughout life, with little modification, the shape and function which they have in the nauplius stage, and must, in all likelihood, be regarded as approximating to those of the primitive Crustacea.
On this view, the nauplius, while no longer regarded as reproducing an ancestral type, does not altogether lose its phylogenetic significance.