(c) The head is deuterognathous—that is to say, there is only one prosthomere, and accordingly the first and only pair of hemignaths is developed by adaptation of the appendages of the second somite.
(d) The appendages of the third somite (second post-oral) are clawless oral papillae.
(e) The rest of the somites carry equi-formal simple appendages, consisting of a corm or axis tipped with two chitinous claws and devoid of rami.
(f) The segmentation of the body is anomomeristic, there being no fixed number of somites characterizing all the forms included.
(g) The pair of eyes situated on the prosthomere are not of the Euarthropod type, but resemble those of Chaetopods (hence Nereid-ophthalmous).
(h) The muscles of the body-wall and gut do not consist of transversely-striped muscular fibre, but of the unstriped tissue observed also in Chaetopoda.
(i) A pair of coelomoducts is developed in every somite including the prosthomere, in which alone it atrophies in later development.
(j) The ventral nerve-cords are widely separated—in fact, lateral in position.
(k) There are no masses of nerve-cells forming a ganglion (neuromere) in each somite. (In this respect the Protarthropoda are at a lower stage than most of the existing Chaetopoda.)
(l) The genital ducts are formed by the enlargement of the coelomoducts of the penultimate somite.