Although several plans for organizing an organism emerged from the primordial soup, one of the most common to survive the early competition takes the form of a 3-legged and 3-armed animal
1, usually with a siliceous outer casing of plate or shell. This basic unit is called a
node. By itself the node is a complete organism, however in all but the least advanced species the sexual organelles remain unused so long as it travels solo.
Two compatible nodes may fuse at a leg tip
2. The remaining limbs remain free, forming a 4-legged and 6-armed superorganism called a
dinode (or in some taxonomies a
duple). Electrodynamic blood analogue flows freely across the point of juncture, sharing nutrition between nodes; a triggered adaptive process encourages the individuals' immune systems to quickly recognize and accept the other's flesh
3; not too long after that nerve analogues extend across the connection, allowing each partner access to the other's senses and even memories. There is not a full "mind meld" as each member retains its own core cortex, but various data can be transmitted easily along superconducting silicon nerves. Different taxa exhibit a number of variations in the nature of this coupling, with some retaining a flexible joint and others thickening the dermal plates and locking the limbs into place. After a sufficient period together, the tissues at the conjunction can and will grow past and into each other, blurring the boundary line. Traveling stem cell analogues floating in the bloodstream may congregate and repair tissues in the 'other' node to further erase a genetic notion of identity.
The dinode is a highly stable form in most organisms; there is little advantage to separating again when one could be part of a superorganism with twice the size, twice the sensory organelles, and twice the feeding capacity. The dinode can be further extended as a
polynode by combining another node or dinode at a free leg tip. Polynodes can potentially form long chains or, more rarely, rings. Motor coordination amongst the nodes becomes increasingly difficult as the distance between them increases: even superconducting silicon nerves have a speed limit, while the cortex of each node tends to be proprietary about its own muscles. For this reason most taxa that form numerically large polynodes tend to be reef builders, with the oldest connected nodes secreting thicker and thicker shells until they become fully sessile while younger portions of the chain dangle/flail/sway outward more actively seeking both nutrition and more joints for the colony.
As polynode chains extend in length, the sex organelles have remained mature but quiescent. They will only activate fully under the condition that at least one node of the superorganism forms a chimeric joining at all three leg tips
4. The affected node(s) produces a hormone that stimulates gamete production throughout the entire superorganism. Genetically the nodes are diploid and require the combination of only two gametes for a complete offspring no matter how many nodes are part of the superorganism. In the vast majority of species these gametes have no size or structural distinction – thus no sexes as humans know them – except that chemical coatings prevent gametes from the same parent node from combining with each other as a zygote. These same coatings also prevent a fused zygote from implanting and incubating within either parent node (or genetically identical repaired tissue).
Both gamete and zygote stages float freely through the blood analogue "seeking" proper conditions to continue growing. A growing zygote or blastula makes its way to the
placental-cambium layer of a non-parental node's dermis where it forms a small cyst and begins growing just beneath the outer plates or shell. It is not unknown for this stage to fail due to radiation exposure before it is fully developed. As maturation continues, the offspring will "dig out" a larger space for itself from the hard outer material in addition to pushing the softer inner tissues aside. Eventually a new node "hatches" by tearing its way through the carapace, leaving a proportional hole in the gestational node. Since there is a tendency for numerous fetal nodes to develop simultaneously, the accumulated injuries can be fatal to the parental superorganism, either directly or via necrosis of the compromised tissue.
There is a significant evolutionary divide comparable to Terran taxonomic orders, or even kingdoms, between organisms that form dinodes quickly prior to reaching sexual maturity, and those that delay until adolescence or later
5. (Future) fossil records indicate that the former, enjoying the advantages of powerful dinode bodies
6, tended to dominate early history. In these types it is usual for dinodes and single nodes of a given taxon to be present in the free moving population with about equal proportion. On the other hand, the types that remain single for a longer period of their childhood appear to have experienced more evolutionary pressure to develop advanced signaling, emotions, and "intelligence" in order to inform their quest for mates as well as for living life generally.
Notes
1) Barring amputating injuries or polydactyl cancers, of course. Has the regenerative capacity of our life forms been decided?
2) Cue the evolutionary arms race as predatory nodes mimic connection signaling in order to lure prey.
3) Cue the evolutionary arms race as infectious life takes advantage of this trial period to spread from host to host.
4) Some advanced organisms may exploit this fact as a form of birth control, delaying the formation of a reproductive trigger joint until certain conditions are met.
5) Only a handful of taxa go full polynode prior to reproductive maturity; it seems that was an evolutionary dead end although some of the reef builder types are prolific if not very advanced.
6) No doubt some species grew very large to maximize this advantage. Dinode-saurs
7, we could call them.
7) Honest to goodness, I did not invent the name just for that pun. The pun came to me much later while I was writing.