The later stages in the differentiation of the ventral cord are, so far as is known, very similar throughout the Crustacea. The ventral cord is, as has been stated, at first unsegmented ([fig. 241] A, vg), but soon becomes divided by a series of constrictions into as many ganglia as there are pairs of appendages or segments ([fig. 241] B, vg).
There appears either on the ventral side (Oniscus) or in the centre (Astacus, Palæmon) of the two halves of each segment or ganglion a space filled with finely punctuated material, which is the commencement of the commissural portion of the cords. The commissural tissue soon becomes continuous through the length of the ventral cord, and is also prolonged into the supra-œsophageal ganglia.
After the formation of the commissural tissue the remaining cells of the cord form the true ganglion cells. A gradual separation of the ganglia next takes place, and the cells become confined to the ganglia, which are finally only connected by a double band of commissural tissue. The commissural tissue not only gives rise to the longitudinal cords connecting the successive ganglia, but also to the transverse commissures which unite the two halves of the individual ganglia.
The ganglia usually, if not always, appear at first to correspond in number with the segments, and the smaller number so often present in the adult is due to the coalescence of originally distinct ganglia.
Organs of special sense. Comparatively little is known on this head. The compound eyes are developed from the coalescence of two structures, both however epiblastic, viz. (1) part of the superficial epiblast of the procephalic lobes; (2) part of the supra-œsophageal ganglia. The former gives rise to the corneal lenses, the crystalline cones, and the pigment surrounding them; the latter to the rhabdoms and the cells which encircle them. Between these two parts a mesoblastic pigment is interposed.
Of the development of the auditory and olfactory organs almost nothing is known.
Dorsal organ. In a considerable number of the Malacostraca and Branchiopoda a peculiar organ is developed from the epiblast in the anterior dorsal region. This organ has been called the dorsal organ. It appears to be of a glandular nature, and is usually very large in the embryo or larva and disappears in the adult; but in some Branchiopoda it persists through life. In most cases it is unpaired, but in some instances a paired organ appears to take its place.
Various views as to its nature have been put forward. There is but little doubt of its being glandular, and it is possible that it is a provisional renal organ, though so far as I know concretions have not yet been found in it.
Its development has been most fully studied in the Isopoda.