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.
The development usually begins with a nauplius stage (Sars, 1896, 1900).
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.
The various larval forms, especially the nauplius and zoea, were supposed to reproduce, more or less closely, the actual structure of ancestral types.
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.
A hero Nauplius took part in the Argonautic expedition; another was king of Euboea.
As development proceeds, the body of the nauplius elongates, and indications of segmentation begin to appear in its posterior part.
In most of the Crustacea which hatch at a later stage there is, as already mentioned, more or less clear evidence of an embryonic nauplius stage.
It seems certain, therefore, that the possession of a nauplius larva must be regarded as a very primitive character of the Crustacean stock.
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.