Biology · Animal Diversity & Body Plans
Echinoderms, chordates and deuterostome relationships
A sea star's adult body is arranged around a centre, while a lancelet has a recognisable front-to-rear axis. Both appear in the traditional deuterostome comparison. Their relationships cannot be read simply by counting arms or looking for a backbone.
Use Phylogeny, Classification & the Tree of Life to read common ancestors and nested groups. Earth, Origins & the History of Life explains fossil and dating limits, and Development, Differentiation & Stem Cells distinguishes development within one life from evolution across generations. Here, a clade contains a common ancestor and all its descendants; a shared shape alone does not establish a clade.
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Connect echinoderm and chordate features to selected relationships while recognising developmental changes and uncertainty in deeper branching.
Distinguish a supported subgroup from a deeper question
Deuterostomia traditionally groups chordates with Ambulacraria, the branch containing echinoderms and hemichordates. Comparative genomes have supported this arrangement and identified shared features. However, some phylogenomic analyses question whether the combined deuterostome grouping is a clade. Deep relationships among the major bilateral animal branches remain an active research problem.
This uncertainty does not erase evidence for narrower groups. Echinoderms and hemichordates form the ambulacrarian comparison, while chordates include lancelets, tunicates and vertebrates. Hemichordates have pharyngeal slits but lack the chordate notochord. One shared structure need not imply that an animal belongs inside Chordata.
The developmental name is historical. Mouth and anus formation, cleavage and cavity development vary among species, as the earlier lesson explained. A sea star is not placed in this comparison because every developmental event matches a universal rule. Relationships are evaluated from multiple independent features and molecular evidence.
A radial adult can belong to Bilateria
Echinoderms include sea stars, brittle stars, sea urchins, sea cucumbers and crinoids. Their skeleton contains internal mineralised elements. Many adults have a five-part arrangement, but larval and adult forms differ, and adult symmetry can also be modified. Bilateria names a lineage; it does not promise that every adult descendant retains a visibly bilateral outline.
A water-vascular system connects fluid-filled canals to tube feet. In a selected sea-star organisation, a ring connects to radial canals and then to tube feet. Local fluid movement, muscles and attachment structures contribute to tube-foot activity. The system can support movement, feeding and other exchanges, depending on the organism and structure.
The network model traces hydraulic connections, not digestive transport or blood circulation. It omits most real branches and shows only two tube feet per arm. Their number is a drawing choice. The radial arrangement expresses repeated access to connected structures, without turning the whole animal into a simple pipe pump.
A ring connects repeated tube-foot routes
Read the diagram in words
A ring of radius 1 connects to five radial canals ending at radius 3. Two depicted tube feet attach to each canal at radius 2.5. There are 10 depicted feet and 15 listed hydraulic connections: five ring-to-canal and ten canal-to-foot. The edges do not encode digestion or descent.
| Identifier | Structure or interpretation | Centre x | Centre y | Radius |
|---|---|---|---|---|
| ring | Ring canal | 0 | 0 | 1 |
| Identifier | Structure or interpretation | Angle (degrees) | Start x | Start y | End x | End y |
|---|---|---|---|---|---|---|
| radial-1 | Radial canal 1 | 90 | 6.123233995736766e-17 | 1 | 1.8369701987210297e-16 | 3 |
| radial-2 | Radial canal 2 | 162 | -0.9510565162951535 | 0.3090169943749475 | -2.8531695488854605 | 0.9270509831248426 |
| radial-3 | Radial canal 3 | 234 | -0.5877852522924732 | -0.8090169943749473 | -1.7633557568774196 | -2.427050983124842 |
| radial-4 | Radial canal 4 | 306 | 0.5877852522924729 | -0.8090169943749476 | 1.7633557568774187 | -2.427050983124843 |
| radial-5 | Radial canal 5 | 378 | 0.9510565162951536 | 0.3090169943749472 | 2.853169548885461 | 0.9270509831248415 |
| Identifier | Structure or interpretation | Radial canal identifier | Attachment x | Attachment y | Tip x | Tip y |
|---|---|---|---|---|---|---|
| foot-1 | Selected tube foot 1 | radial-1 | 1.5308084989341916e-16 | 2.5 | 0.4000000000000002 | 2.5 |
| foot-2 | Selected tube foot 2 | radial-1 | 1.5308084989341916e-16 | 2.5 | -0.39999999999999986 | 2.5 |
| foot-3 | Selected tube foot 3 | radial-2 | -2.3776412907378837 | 0.7725424859373687 | -2.2540344929879046 | 1.1529650924554302 |
| foot-4 | Selected tube foot 4 | radial-2 | -2.3776412907378837 | 0.7725424859373687 | -2.5012480884878627 | 0.3921198794193073 |
| foot-5 | Selected tube foot 5 | radial-3 | -1.4694631307311832 | -2.022542485937368 | -1.7930699284811622 | -1.787428385020379 |
| foot-6 | Selected tube foot 6 | radial-3 | -1.4694631307311832 | -2.022542485937368 | -1.1458563329812042 | -2.2576565868543574 |
| foot-7 | Selected tube foot 7 | radial-4 | 1.4694631307311823 | -2.022542485937369 | 1.1458563329812033 | -2.2576565868543583 |
| foot-8 | Selected tube foot 8 | radial-4 | 1.4694631307311823 | -2.022542485937369 | 1.7930699284811613 | -1.7874283850203798 |
| foot-9 | Selected tube foot 9 | radial-5 | 2.377641290737884 | 0.772542485937368 | 2.501248088487863 | 0.3921198794193065 |
| foot-10 | Selected tube foot 10 | radial-5 | 2.377641290737884 | 0.772542485937368 | 2.254034492987905 | 1.1529650924554296 |
| First identifier | First structure or lineage | Second identifier | Second structure or lineage | Relationship meaning | Structure or interpretation |
|---|---|---|---|---|---|
| ring | Ring canal | radial-1 | Radial canal 1 | Hydraulic connection | Fluid connection |
| ring | Ring canal | radial-2 | Radial canal 2 | Hydraulic connection | Fluid connection |
| ring | Ring canal | radial-3 | Radial canal 3 | Hydraulic connection | Fluid connection |
| ring | Ring canal | radial-4 | Radial canal 4 | Hydraulic connection | Fluid connection |
| ring | Ring canal | radial-5 | Radial canal 5 | Hydraulic connection | Fluid connection |
| radial-1 | Radial canal 1 | foot-1 | Selected tube foot 1 | Hydraulic connection | Fluid connection to selected tube foot |
| radial-1 | Radial canal 1 | foot-2 | Selected tube foot 2 | Hydraulic connection | Fluid connection to selected tube foot |
| radial-2 | Radial canal 2 | foot-3 | Selected tube foot 3 | Hydraulic connection | Fluid connection to selected tube foot |
| radial-2 | Radial canal 2 | foot-4 | Selected tube foot 4 | Hydraulic connection | Fluid connection to selected tube foot |
| radial-3 | Radial canal 3 | foot-5 | Selected tube foot 5 | Hydraulic connection | Fluid connection to selected tube foot |
| radial-3 | Radial canal 3 | foot-6 | Selected tube foot 6 | Hydraulic connection | Fluid connection to selected tube foot |
| radial-4 | Radial canal 4 | foot-7 | Selected tube foot 7 | Hydraulic connection | Fluid connection to selected tube foot |
| radial-4 | Radial canal 4 | foot-8 | Selected tube foot 8 | Hydraulic connection | Fluid connection to selected tube foot |
| radial-5 | Radial canal 5 | foot-9 | Selected tube foot 9 | Hydraulic connection | Fluid connection to selected tube foot |
| radial-5 | Radial canal 5 | foot-10 | Selected tube foot 10 | Hydraulic connection | Fluid connection to selected tube foot |
| Label identifier | Structure or interpretation |
|---|---|
| oral | Selected oral-side arrangement |
| system | Water-vascular connections |
| feet | Two feet per arm shown; real counts not assigned |
| edges | Connections do not assign continuous one-way flow |
| scale | Schematic radial positions; no measured anatomy |
Selected sea-star network; one ring, five radial canals and two schematic tube feet per canal. Actual numbers, detailed valves, ampullae, muscles and pressure dynamics are omitted.
Drawing units
Chordate features can appear at different stages
A notochord is a supporting rod along the body axis. A dorsal hollow nerve cord lies above it, and the gut lies below it in the selected side-view comparison. Chordates also show pharyngeal structures, a tail extending beyond the anus at some stage, and an endostyle or related thyroid organisation.
These features are modified across development and lineages. A lancelet retains the characteristic axial organisation as an adult. Many attached tunicate adults differ markedly from their swimming larvae. Vertebral elements develop around the notochord in vertebrates, but the notochord is not simply absent from every adult vertebrate; its persistence and derivatives vary.
Pharyngeal structures likewise have different outcomes. A human embryo does not have a functional fish gill apparatus. Shared developmental regions are transformed into different structures. Recognising homology does not mean that a mammal briefly becomes a fish, nor that a tunicate larva is the direct ancestor of a vertebrate.
Chordate structures follow an anatomical axis
Read the diagram in words
With positive y dorsal, the hollow nerve-cord centreline at 1.5 lies above the notochord at 1, which lies above the main digestive route at 0. Three selected slits occupy the pharyngeal region, with an endostyle on its floor. The body extends from anus x = 8 to tail endpoint x = 10.
| Axis or transformation | x coordinate | y coordinate |
|---|---|---|
| anterior | 0 | 0 |
| posterior | 10 | 0 |
| dorsal | 5 | 3 |
| ventral | 5 | -2 |
Posterior along the body axis
Dorsal
| Path identifier | Point order | x coordinate | y coordinate |
|---|---|---|---|
| Selected body boundary, open at the mouth | 1 | 0.8 | 0.6 |
| Selected body boundary, open at the mouth | 2 | 0.4 | 0.8 |
| Selected body boundary, open at the mouth | 3 | 0 | 1 |
| Selected body boundary, open at the mouth | 4 | 0 | 2 |
| Selected body boundary, open at the mouth | 5 | 4 | 2.6 |
| Selected body boundary, open at the mouth | 6 | 8 | 2.4 |
| Selected body boundary, open at the mouth | 7 | 10 | 2 |
| Selected body boundary, open at the mouth | 8 | 10 | -1 |
| Selected body boundary, open at the mouth | 9 | 8 | -1 |
| Selected body boundary, open at the mouth | 10 | 4 | -1.4 |
| Selected body boundary, open at the mouth | 11 | 0.8 | -0.8 |
| Selected body boundary, open at the mouth | 12 | 0.8 | 0.2 |
| Identifier | Structure or interpretation | Geometry type | x coordinate | y coordinate | Width | Height | Centreline y |
|---|---|---|---|---|---|---|---|
| nerve | Dorsal hollow nerve cord | Paired boundary lines | — | — | — | — | 1.5 |
| notochord | Notochord | Paired boundary lines | — | — | — | — | 1 |
| pharynx | Pharyngeal region | Rectangle | 1 | -0.6 | 2 | 1.2 | — |
| gut | Digestive route | Open polyline | — | — | — | — | — |
| endostyle | Endostyle on pharyngeal floor | Line | — | — | — | — | — |
| Path identifier | Point order | x coordinate | y coordinate |
|---|---|---|---|
| nerve · Upper boundary | 1 | 0.4 | 1.7 |
| nerve · Upper boundary | 2 | 9.3 | 1.7 |
| nerve · Lower boundary | 1 | 0.4 | 1.3 |
| nerve · Lower boundary | 2 | 9.3 | 1.3 |
| notochord · Upper boundary | 1 | 0.3 | 1.1 |
| notochord · Upper boundary | 2 | 9.7 | 1.1 |
| notochord · Lower boundary | 1 | 0.3 | 0.9 |
| notochord · Lower boundary | 2 | 9.7 | 0.9 |
| gut | 1 | 0.8 | 0.4 |
| gut | 2 | 1 | 0.4 |
| gut | 3 | 3 | 0 |
| gut | 4 | 7.7 | 0 |
| gut | 5 | 8 | -1 |
| endostyle | 1 | 1.3 | -0.6 |
| endostyle | 2 | 2.8 | -0.6 |
| Identifier | Structure or interpretation | Start x | Start y | End x | End y |
|---|---|---|---|---|---|
| slit-1 | Selected pharyngeal slit | 1.5 | 0.3 | 1.5 | -0.4 |
| slit-2 | Selected pharyngeal slit | 2 | 0.3 | 2 | -0.4 |
| slit-3 | Selected pharyngeal slit | 2.5 | 0.3 | 2.5 | -0.4 |
| Structure or interpretation | x coordinate | y coordinate |
|---|---|---|
| Mouth opening | 0.8 | 0.4 |
| Anus position | 8 | -1 |
| Structure or interpretation | Start x | Start y | End x | End y |
|---|---|---|---|---|
| Mouth opening | 0.8 | 0.2 | 0.8 | 0.6 |
| Structure or interpretation | Start position | End position |
|---|---|---|
| Body extends beyond the anus | 8 | 10 |
| First identifier | Second identifier | Relationship meaning | Structure or interpretation |
|---|---|---|---|
| nerve | notochord | Dorsal-to relation | Nerve cord lies dorsal to notochord |
| notochord | gut | Dorsal-to relation | Notochord lies dorsal to the main gut route |
| Label identifier | Structure or interpretation |
|---|---|
| tail | Post-anal by position |
| slits | Openings from the pharyngeal region; surrounding chamber omitted |
| scale | Selected positions and proportions; no developmental sequence |
Original lancelet-like axial comparison. Detailed tissues, vessels, muscles, atrium and atriopore are omitted; pharyngeal slits are not a direct seawater outlet assigned in this schematic.
Drawing units
Worked example
Follow one water-vascular connection and one axial relation
The constructed sea-star network has one ring, five radial canals and two tube feet attached to each radial canal. It therefore contains 10 tube feet. The listed connections include five ring-to-radial links and 10 radial-to-foot links, totalling 15. These are hydraulic connections, not parent–offspring relationships.
In the selected chordate side view, a nerve-cord centreline lies at y = 1.5, the notochord at y = 1.0 and the digestive route mainly at y = 0. Positive y is dorsal. The nerve cord is therefore dorsal to the notochord, which is dorsal to the gut.
The anus is assigned x = 8, and the body tail extends to x = 10. The two-unit region beyond the anus is post-anal by position. A different organism could modify or lose that adult region; its developmental history would still matter for a chordate comparison.
Check your understanding
Does an adult's five-part outline exclude a sea star from Bilateria?
Show reasoning
No. Bilateria is an evolutionary lineage. Echinoderm development includes bilateral larval organisation, and adult form is modified. Adult symmetry alone does not rewrite ancestry.
Why is the sea-star diagram not a digestive flow map?
Show reasoning
Its edges connect a water-vascular ring, radial canals and tube feet. They denote hydraulic connection. Digestive structures and nutrient circulation are different systems and are omitted.
If the notochord centreline is y = 1.0 and the nerve cord is y = 1.5, which is dorsal in the stated view?
Show reasoning
The nerve cord is dorsal because positive y is explicitly assigned dorsal. The relation relies on the anatomical axis convention; rotating the page would not change the organism's dorsal side.
What to carry forward
Use stage-specific structure and nested relationships together; uncertainty at a deep node is not uncertainty about every subgroup.
References
Resources supporting this lesson’s explanations and examples.
- Biology 2e: animal organisation, diversity and vertebrate relationships (2018; selected sections in chapters 27–29)Mary Ann Clark, Matthew Douglas and Jung ChoiTextbook sections
- Hemichordate genomes and deuterostome origins (Nature 527:459–465 (2015); doi:10.1038/nature16150)Oleg Simakov and colleaguesPrimary research
- Lack of support for Deuterostomia prompts reinterpretation of the first Bilateria (Science Advances 7:eabe2741 (2021); doi:10.1126/sciadv.abe2741)Paschalia Kapli and colleaguesPrimary research · PDF