REMARK : When we, in the course of the ensuing discussions, quote a certain promorph (= stereometric basic form or promorphological category), we will set it as a LINK. This link will bring the reader to the relevant place within the Promorphological System, so that he or she can orient him- or herself as to where that category is situated within the System.
The Metamers or sequential parts, i.e. fourth-order form individuals, show a fairly different morphological behavior, as to whether they occur as actual bionts (i.e. free living organic form individuals), and thus as isolated metamers, or as subordinated form individuals of persons. In the former case their form diversity is very great, in the latter more limited.
When the metamers occur as actual bionts, as we see it in Molluscs, non-articulated worms and in some lower plants, they can, in line with the number and connection of the constituent antimers, either adopt the Eudipleural form, as we see it most often, or assume some other promorph. Because a metamer consists by definition of two or more antimers, its promorph can never be that of the Anaxonia, Homaxonia, and Monaxonia. In all cases the promorph of metamers must belong to the Heteraxonia, and among these only the monaxon form and the homopolar stauraxonic form are excluded, because the former does not possess antimers and so cannot itself be a metamer, and the latter is already a whole of two metamers.
In Echinoderms (starfishes, sea-urchins and the like), which always occur as single metamers, we find a promorph belonging either to the (Stauraxonia) heteropola homostaura, or to the (Heterostaura) allopola amphipleura.
In Molluscs, which, as actual bionts, all are just metamers, i.e. they, like all echinoderms, occur as single metamers, the shell-bearing representatives, like snails, adopt a spiral form and must promorphologically be classified as Spiraxonia isogonia (equiangular spirals). They are pyramidal or conical forms having their main axis spirally curved according to the equiangular spiral, most conspicuous so, in their shells.
The P r o t a x o n i a, i.e. bodies possessing one conspicuous axis (the "main axis"), sometimes having this axis in addition to some lesser conspicuous axes (cross axes), consist of three divisions : Monaxonia (bodies possessing only one axis), Stauraxonia (bodies, possessing in addition to a main axis, some other axes (cross axes) perpendicular to that main axis), and Spiraxonia (bodies, with their main axis spirally curved).
Other Molluscs, like slugs and most of the recent cephalopods (octopus, squid) assume a dipleural ( Dipleura ) promorph.
When on the other hand, the metamers occur as subordinated form individuals of a person (fifth-order form individual), as in all vertebrates and arthropods (insects, shrimps and the like), and in many higher plants, we observe that the number of different promorphs, which are realized in them, is more limited. Here we see that their promorph is often the same as that of the person which they constitute. So in the regular flowers of higher plants we find the homostauric form ((Stauraxonia) heteropola homostaura, regular pyramids), in the bilateral symmetric animals and plant off-shoots the heterostauric form ((Stauraxonia) heteropola heterostaura, irregular pyramids). But there are exceptions to this rule (that the metamers have the same promorph as the person which they constitute), like for instance the different metamers ('leaf rings') of a single higher plant flower. The metamers constituting such a flower often have different promorphs. So we often see in the five-fold flowers of the Papilionaceans, Labiates, etc., that the promorph of the metamer of the calyx is a five-fold regular pyramid (Homostaura), while the other metamers of the flower have as their promorph half a ten-fold amphitect pyramid (Pentamphipleura). In the same way we see in many annelids (earthworms and the like) the promorph of most metamers of the body to belong to the Eutetrapleura, while the front metamers, especially those of the head, belong promorphologically to the Eudipleura. In Taenia (a tape worm) is, the other way round, its head tetractinote ((Stauraxonia heteropola homostaura) Isopola tetractinota), regular four-fold pyramid), while the metamers that follow (Proglottids) are diphragmic ((Stauraxonia heteropola heterostaura autopola) Orthostaura diphragma). In these cases where several metamers of one and the same person have different promorphs, always the highest promorph, i.e. the most differentiated promorph, is the promorph of the whole person. This is also geometrically correct, because the symmetry of a whole object is equal to the symmetry of that part that has the lowest symmetry (when compared with that of the other parts).
Because the promorph of the metamers, as also that of the persons, can be different in different life-stadia of an individual, this stadium should be indicated in every promorphological assessment.
In the great majority of all metamers in the animal as well as in the plant kingdom, the basic form is half an amphitect pyramid ( Zeugita ). Thereby it is most often the isosceles pyramid ( Zygopleura tetrapleura and Zygopleura dipleura ), further half an amphitect pyramid with (after halving) three, five or more sides ( Amphipleura ) (in irregular echinoderms). But also many metamers have as promorph the regular pyramid (Stauraxonia heteropola homostaura), as we see in regular echinoderms that are as actual bionts each for themselves a single metamer.
All what has been said of the promorph of the metamers, which constitute a person, is equally valid with respect to the e p i m e r s, which, in a similar way (as sequential parts) constitute organs and cells. Just like the metamers normally assume the promorph of the person, the epimers assume the promorph of the organs and cells which they compose, i.e. of which they are sequential parts.
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