(g) The two eyes of Chaetopod structure have disappeared, and are replaced by the Euarthropod eyes.
(h) The muscles in all parts of the body consist of striped muscular fibre, never of unstriped muscular tissue.
(i) The coelomoducts are suppressed in most somites, and retained only as the single pair of genital ducts (very rarely more numerous) and in some also as the excretory glands (one or two pairs).
(j) The ventral nerve-cords approach one another in the mid-ventral line behind the mouth.
(k) The nerve-cells of the ventral nerve cords are segregated as paired ganglia in each somite, often united by meristic dislocation into composite ganglia.
(l) The genital ducts may be the coelomoducts of the penultimate or ante-penultimate or adjacent somite, or of a somite placed near the middle of the series, or of a somite far forward in the series.
Class 1 (of the Euarthropoda).—Diplopoda.
The head has but one prosthomere (monoprosthomerous), and is accordingly deuterognathous. This carries short-jointed antennae (in one case bi-ramose) and eyes, the structure and development of which require further elucidation. Only one somite following the first post-oral or mandibular segment has its appendages modified as jaws.
The somites of the body, except in Pauropus, either fuse after early development and form double somites with two pairs of appendages (Julus, &c.), or present legless and leg-bearing somites alternating.
Somites, anomomeristic, from 12 to 150 in the post-cephalic series.