Starfish and Tardigrades are All Head

Adult starfish (sea stars), at least. Along with adults of some other echinoderms: sea urchins (echinoids) and sea lilies (crinoids).

Starfish Are Heads--Just Heads | Scientific American
noting an article in Nature magazine: the animals are actually almost all head and no trunk

This seems like an odd sort of thing, because starfish don't seem to have any head: just a body that is the center of the arms, usually 5, sometimes more. Furthermore, adult starfish have radial symmetry, like other echinoderms, while their larval phases have bilateral symmetry.

This result was found by examining the expression of genes involved in nose-to-tail (anteroposterior) patterning, genes that include the famous Hox genes. The proteins from their expression then control the expression of other genes, producing identity along this axis. This nose-to-tail patterning system is found over most of the animal kingdom, and most of the time, it is for patterning the main body axis.

But starfish are different, this research shows.

The most forward expressed genes are actually expressed around the central tubes of the water vascular system: a ring in the "body" with radial tubes in the arms. These tubes are on the ventral (downward) side of the animal. Next most forward genes are expressed at the tube feet, a little sideways from the radial tubes, and next forward, but still in the head, genes are expressed a little bit further sideways.

So these tubes are "frontmost" and outward from these tubes, like sideways in the arms, is "backward" from "frontmost", but still "head".

Looking at other echindoderms, sea urchins are like starfish with their arms pulled upward, surrounding the "body". Sea lilies are like upside-down starfish on stalks.

Working out how this body plan emerged is a very challenging task, and some fossils of early echinoderms may have some clues.

Adding to this weirdness is how starfish grow. Most starfish hatch into a larval form called the  Bipinnaria that is first jellybean-shaped then grows some projections

When it initially forms, the entire body is covered by cilia, but as it grows, these become confined to a narrow band forming a number of loops over the body surface. A pair of short, stubby arms soon develop on the body, with the ciliated bands extending into them.

... Eventually, three additional arms develop at the front end of the larva; at this point it becomes a brachiolaria.

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The second larval form is the  Brachiolaria

The brachiolaria develops from the bipinnaria larva when the latter grows three short arms at the underside of its anterior end. These arms each bear sticky cells at the tip, and they surround an adhesive sucker. The larva soon sinks to the bottom, attaching itself to the substrate, firstly with the tips of the arms, and then with the sucker. Once attached, it begins to metamorphose into the adult form.

The adult starfish develops only from the hind-part of the larva, away from the sucker. It is from this part that the arms of the adult grow, with the larval arms eventually degenerating and disappearing. The digestive system of the larva also degenerates, and is almost entirely rebuilt. A new mouth forming on the left side of the body, which eventually becomes the lower, or oral, surface of the adult. Similarly, a new anus forms on the right side, which becomes the upper, or aboral, surface.

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Some starfish skip the brachiolaria phase and some skip both phases, hatching as miniature adults.

Marine Invertebrate Larvae and  Larva

Many other marine invertebrates also have oddly-named larvae, and those larvae also often look very different from the adults.

• Starfish - bipinnaria then brachiolaria
• Brittle stars - ophiopluteus
• Sea urchins - echinopluteus
• Sea cucumbers - auricularia then doliolaria then pentacularia
• Sea lilies - vitellaria
• Hemichordate worms - tornaria (looks much like bipinnaria)
• Sea squirts - tadpole
• Mollusks - trochophore then often veliger
• Annelids - trochophore
• Crustaceans - nauplius (becomes the head) sometimes then zoea
• Cnidarians - planula
• Ctenophores - cydippid

The evolution of early-animal larval phases continues to be a major controversy, with two rival theories: larva-first and adult-first.

Larva-first is like land vertebrates, growing into full-scale land animals from a fishlike phase. Tadpoles, larval frogs, have fishlike features like a lateral line, gills, and tails for swimming. Some frogs do their larval phase as embryos, hatching as miniature adults, and amniotes (reptiles, birds, mammals) do the same.

Adult-first is like four-stage insects (holometabolous ones). Their wormlike larvae are a specialization of their immature phase. Their pupa phase is a sort of catch-up phase for making the adult phase.

Multiple origins of feeding head larvae by the Early Cambrian - Multiple origins of feeding head larvae by the Early Cambrian

Like the crustacean nauplius phase, which becomes the head as the animal grows, adding segments to its rear end.

Most segmented animals add new segments on their rear ends, though some insects, like flies, lay down their segments all at once, making them long-germ-band insects rather than short-germ-band ones, like beetles and most others.

I'll now turn to the other all-head animal: the tardigrade. This was reported on some years back.

Tardigrades or water bears or moss piglets are small animals, typically around 0.5 millimeters long,

Their closest relatives are arthropods and onychophorans (velvet worms), grouped together as Panarthropoda, sharing a body plan of segments with at least some of their segments growing legs.

Tardigrades have five segments: a head, three trunk segments, and a tail-end segment, with a pair of stubby legs on all of the segments except for the head.

• The metameric pattern of Hypsibius dujardini(Eutardigrada) and its relationship to that of other panarthropods | Frontiers in Zoology | Full Text
• Neural Markers Reveal a One-Segmented Head in Tardigrades (Water Bears) | PLOS ONE
• Segmentation in Tardigrada and diversification of segmental patterns in Panarthropoda - ScienceDirect with Segmentation in Tardigrada and diversification of segmental patterns in Panarthropoda - SmithGoldstein2017.pdf
• The Compact Body Plan of Tardigrades Evolved by the Loss of a Large Body Region - PubMed from The Compact Body Plan of Tardigrades Evolved by the Loss of a Large Body Region: Current Biology

Most other panarthropods have many more segments, and in them, the first few segments' legs became mouthparts and sometimes antennae.

Looking at the nose-to-tail patterning system, the expression patterns of its genes have proved very valuable in identifying the homologies of panarthropod segments.

All but tardigrades have more than five segments when fully grown, sometimes a lot more, and their heads typically have 3 or 6 segments, with legs specialized as mouthparts or antennae when present.

Fully grown? Many crustaceans hatch as a nauplius larva, with four segments, a frontmost one and three ones with legs. As the animal grows, it adds segments on its rear end, with its legs becoming antennae and mouthparts. It thus hatches as a head that later grows a body.

Likewise, by homology, a tardigrade is all head except for its rear-end segment.

Note: all body parts assumed paired except for those of the first and last segments. The labrum is a sort of mouthpart flap.

Tardigrade: (none) - (3 walking legs) - (rear-end segment with a pair of legs)

Insect: (eyes, labrum) - (antenna) - (none) - (3 jaw parts) - (3 walking legs) - (around 10 legless segments) - (rear-end segment that may have limbs: cerci)

Crustacean: (eyes, labrum) - (2 antennae) - (3 or more jaw parts) - (lots of walking legs) - ?
Nauplius larva: (eyes, labrum) - (3 swimming legs)

Myriapod: (eyes, labrum) - (antenna) - (none) - (3 jaw parts) - (lots of walking legs) - ?

Arachnid: (eyes, labrum) - (chelicera) - (pedipalp) - (4 walking legs) - ?
Chelicera = a claw-like mouthpart
Pedipalp = a sort-of arm

Onychophoran: (eyes, antennae) - (jaw) - (slime gland) - (lots of walking legs)


So unlike starfish, tardigrades' evolution is much better understood. They make a few segments then decide that they are fully grown.

More:

• Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective - ScienceDirect with PDF at "View Open Manuscript"
• Analyses of nervous system patterning genes in the tardigrade Hypsibius exemplaris illuminate the evolution of panarthropod brains | EvoDevo | Full Text
• The velvet worm brain unveils homologies and evolutionary novelties across panarthropods | BMC Biology | Full Text

Panarthropods' limbs
Labrum = unpaired mouthpart flap, everything else is paired
Radiodont = includes Anomalocaris and similar early arthropods.
GA = great appendage
Chelicera = claw-like part
Pedipalp = arm-like limb
Nauplius = some crustaceans' hatchlings

Some crustaceans - the decapods - have lost abdominal limbs, much like insects and arachnids.

I am mistaken about  Decapod anatomy - these crustaceans have limbs on all of their segments. Lobsters, crayfish, crabs, and many shrimp are decapods.

1. Labrum (not paired, unlike the others)
2. Antenna 1
3. Antenna 2
4. Mandible (food collector / jaw)
5. Maxilla 1 (food collector / jaw)
6. Maxilla 2 (food collector / jaw)
7. Maxilliped 1 (leg / food collector)
8. Maxilliped 2 (leg / food collector))
9. Maxilliped 3 (leg / food collector))
10. Pereiopod 1 (walking leg, pincers)
11. Pereiopod 2 (walking leg)
12. Pereiopod 3 (walking leg)
13. Pereiopod 4 (walking leg)
14. Pereiopod 5 (walking leg)
15. Pleopod 1 (swimming leg)
16. Pleopod 2 (swimming leg)
17. Pleopod 3 (swimming leg)
18. Pleopod 4 (swimming leg)
19. Pleopod 5 (swimming leg)
20. Uropod (tail fins)
21. Telson (tail fin)

Returning to starfish anatomy, there is a path to their origin, a path that other organisms have taken. Feeding tentacles, tentacles that surround the mouth and that are good for getting food items to there. They thus likely originated at least four times:

• Echinoderms - their arms
• Cephalopods (nautilus, squid, octopus) - initially ten of them, likely from the mollusk foot
• Cnidarians (sea anemones, hydras, jellyfish, ...)
• Ctenophores (comb jellies)

A feeding-tentacle origin of starfish arms is consistent with starfish being all head.