That the buccal gnathobasic parapodia (the mandibles) were in each of the three grades of prosthomerism only developed after the recession of the mouth and the addition of one, of two, or of three post-oral somites to the prae-oral region had taken place.
The mandibular parapodia may be supposed during the successive stages of this history to have had, from the first, well-developed rami (one or two) of a palp-like form, so that the change required when the mouth passed away from them would merely consist in the suppression of the gnathobase.
The eyes in some Crustacea are mounted on articulated stalks, and from the fact that they can after injury be replaced by antennalike appendages it is inferred that they represent the parapodia of the most anterior prosthomere.
The mesoderm becomes segmented, and the parapodia subsequently develop from before backwards; but almost all internal traces of segmentation are lost in the adult.
Shell external, globular or ovoid; foot elongated, parapodia separate from ventral surface; genital duct diaulic. Lobiger.
The agreement of the grouping of the somites, of the form of the parapodia (appendages, limbs) in each region, of the position of the genital aperture and operculum, of the position and character of the eyes, and of the powerful post-anal spines not seen in other Arthropods, is very convincing as to the affinity wise suppressed praegenital somite.
Cephalic shield pointed behind; shell internal, chiefly membranous, with calcified nucleus, nautiloid; parapodia forming fins.
Laterally the foot gives rise to a pair of mobile fleshy lobes, the parapodia (ep), which can be thrown up so as to cover in the dorsal surface of the animal.
Setae always present and often very large, much varied in form and very numerous, borne by the dorsal and ventral parapodia (when present).
Where locomotive appendages (the parapodia of the Polychaeta) exist, they are never jointed, as always in the Arthropoda; nor are they modified anteriorly to form jaws, as in that group.