Protostomes vs Deuterostomes: Development Differences

Quick answer: Protostomes and deuterostomes are the two great branches of bilaterally symmetrical animals, divided by early embryonic development. In protostomes (earthworms, insects, snails) the blastopore becomes the mouth; in deuterostomes (starfish, humans) it becomes the anus. They also differ in cleavage pattern and in how the coelom forms.

One of the deepest divisions in the animal kingdom is not between vertebrates and invertebrates, but between two ancient developmental programs: protostomes vs deuterostomes. Every triploblastic, bilaterally symmetrical animal belongs to one of these two super-groups, and the difference is written into the very first hours of embryonic life. This article compares protostome vs deuterostome development across the three classical criteria — the fate of the blastopore, the pattern of cleavage, and the method of coelom formation — with tables, examples, exam tips, MCQs, and FAQs.

• Protostome means "mouth first": the embryonic blastopore develops into the mouth. Deuterostome means "mouth second": the blastopore becomes the anus, and the mouth forms later from a second opening.
• Protostomes show spiral, determinate cleavage; deuterostomes show radial, indeterminate cleavage.
• Protostomes form the coelom by splitting of mesoderm (schizocoely); deuterostomes form it by outpocketing of the gut wall (enterocoely).
• Protostome phyla include Platyhelminthes, Nematoda, Annelida, Arthropoda, and Mollusca.
• Deuterostome phyla are Echinodermata, Hemichordata, and Chordata (which includes all vertebrates, including humans).

What Are Protostomes and Deuterostomes?

The terms come from Greek: protostome means "first mouth" (proto = first, stoma = mouth) and deuterostome means "second mouth" (deutero = second). The names refer to what happens to the blastopore — the first opening that forms in the embryo during gastrulation, when the hollow ball of cells (blastula) folds inward to create the primitive gut (archenteron).

In a protostome embryo, that first opening becomes the mouth of the adult animal. The anus, if one exists, forms later as a second opening. Think of an earthworm or a snail: mouth first, anus second. In a deuterostome embryo, the sequence is reversed: the blastopore becomes the anus, and the mouth breaks through later at the opposite end. Think of a developing human or a starfish larva: anus first, mouth second.

This may sound like a small detail, but it reflects two fundamentally different ways of building an animal body. Along with the blastopore’s fate, protostomes and deuterostomes differ in two more embryonic characters — how their cells divide (cleavage) and how their body cavity forms — and these three differences together define one of the most important splits in animal evolution. Molecular studies of DNA have largely confirmed the split, placing nearly all bilateral animals into either the protostome clade or the deuterostome clade.

Note the scope: this division applies to triploblastic, bilaterally symmetrical animals. Sponges, cnidarians, and ctenophores are neither protostomes nor deuterostomes — they diverged before this split and develop by different rules.

Difference 1: Fate of the Blastopore

During gastrulation, the blastula invaginates to form a double-walled cup. The opening of this cup is the blastopore, and the cavity inside is the archenteron (primitive gut). What the blastopore becomes is the single most famous difference between the two groups:

• Protostomes: the blastopore develops directly into the mouth. The anus forms secondarily, usually by a new opening at the posterior end. In some protostomes (certain flatworms), the blastopore closes and both mouth and anus form anew — but the developmental program is still protostome-like.
• Deuterostomes: the blastopore develops into the anus (or the region near it). The mouth forms later as a secondary opening at the anterior end. This is why the group is called "mouth second."

A simple memory hook: protostome = pro (professional, first) mouth; deuterostome = delayed mouth. Examiners frequently ask "in deuterostomes, the blastopore forms the —" and the one-word answer is anus.

Difference 2: Cleavage Pattern — Spiral vs Radial

Cleavage is the series of rapid cell divisions that turns the fertilized egg into the blastula. The geometry of these divisions differs strikingly between the two groups:

• Protostomes — spiral cleavage: the cells divide at oblique angles, so that each new tier of cells sits in the grooves between the cells of the tier below, like bricks in a wall. Viewed from the top, the cells spiral. Spiral cleavage is also determinate (mosaic): the fate of each early cell is fixed, so removing one blastomere destroys the embryo — each cell’s destiny is already decided.
• Deuterostomes — radial cleavage: the cells divide parallel or perpendicular to the egg’s axis, stacking in neat tiers directly above one another, like floors of a building. Radial cleavage is indeterminate (regulative): early blastomeres are flexible, and each one can still form a complete embryo if separated. This is the developmental basis of identical twins in humans — and a favorite exam connection.

The determinate/indeterminate distinction is tested as often as the spiral/radial one. Remember the pair: spiral = determinate = protostome and radial = indeterminate = deuterostome. If an MCQ gives you one term, you should be able to supply the other two instantly.

Difference 3: Coelom Formation — Schizocoely vs Enterocoely

The third classical difference concerns how the coelom (the fluid-filled body cavity lined by mesoderm) is hollowed out of the embryo’s solid mesoderm:

• Protostomes — schizocoely ("split cavity"): blocks of mesoderm form on either side of the gut, and the coelom opens as a split within each solid mesodermal mass. The mesoderm itself arises from a single special cell (the 4d mesentoblast) near the blastopore. Annelids, arthropods, and molluscs are textbook schizocoelomates.
• Deuterostomes — enterocoely ("gut cavity"): pouches pinch off from the wall of the archenteron (the primitive gut), and the cavities of these pouches become the coelom. The mesoderm is literally budded off from the gut. Echinoderms and chordates are textbook enterocoelomates.

Memory hook: schizocoely = split (both start with ‘s’) = protostome; enterocoely = from the enteron (gut) = deuterostome. This pair completes the classic trio every examiner expects: blastopore fate, cleavage, coelom formation.

Protostome vs Deuterostome: Comparison Table

Feature	Protostomes ("mouth first")	Deuterostomes ("mouth second")
Blastopore becomes	Mouth	Anus
Mouth formation	From blastopore (first opening)	Secondary opening (second)
Cleavage pattern	Spiral (oblique divisions)	Radial (parallel/perpendicular divisions)
Cleavage type	Determinate (mosaic); fate fixed early	Indeterminate (regulative); flexible early cells
Mesoderm origin	From 4d mesentoblast cell near blastopore	From outpocketing of archenteron wall
Coelom formation	Schizocoely (splitting of mesoderm)	Enterocoely (pouching of gut wall)
Nervous system (typical)	Ventral nerve cord, ladder-like	Dorsal nerve cord (chordates); nerve ring (echinoderms)
Phyla	Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca	Echinodermata, Hemichordata, Chordata
Examples	Earthworm, cockroach, snail, Ascaris, Planaria	Starfish, Balanoglossus, fish, frog, human

Which Animals Are Protostomes?

The protostome branch is the larger of the two, containing most animal phyla and the overwhelming majority of animal species (insects alone outnumber everything else). The major protostome phyla, in evolutionary order, are:

• Platyhelminthes (flatworms) — e.g., Planaria, Taenia (tapeworm). Triploblastic but acoelomate, so the coelom criterion does not apply; classified as protostomes by cleavage and molecular data.
• Nematoda (roundworms) — e.g., Ascaris. Pseudocoelomate protostomes with determinate development.
• Annelida (segmented worms) — e.g., earthworm, leech. Classic schizocoelomate protostomes with spiral cleavage.
• Arthropoda (jointed-legged animals) — e.g., cockroach, prawn, spider. The most successful animal phylum, all protostomes.
• Mollusca (soft-bodied animals) — e.g., snail, squid, oyster. Spiral-cleaving schizocoelomates (though the coelom is reduced in most).

Additional smaller protostome phyla include Rotifera, Nemertea (ribbon worms), and Brachiopoda (lamp shells, though their placement has been debated). The unifying thread is the developmental program: spiral determinate cleavage and the mouth-first body plan.

Which Animals Are Deuterostomes?

The deuterostome branch is smaller but includes the most familiar large animals — including us. Only three living phyla belong here:

• Echinodermata (spiny-skinned animals) — e.g., starfish, sea urchin. Radial adults but bilateral larvae; textbook enterocoelomates with radial indeterminate cleavage.
• Hemichordata (acorn worms) — e.g., Balanoglossus. Worm-like marine animals sharing gill slits and a dorsal nerve cord with chordates.
• Chordata (animals with a notochord) — e.g., fish, frog, bird, human. The phylum that produced vertebrates and, ultimately, mammals.

The surprise for most students is that starfish are closer relatives of humans than insects are — both are deuterostomes, while insects are protostomes. This is one of the most quoted illustrations of why developmental and molecular evidence can overturn superficial resemblance.

Exceptions and the Modern View

The three classical differences are ideals, and real embryos sometimes blur them — a point advanced textbooks and competitive exams occasionally probe:

• Some protostomes (e.g., certain nematodes and rotifers) show modified or superficially radial cleavage, and a few deuterostomes show spiral-like patterns. The cleavage rule is strong but not absolute.
• In some protostomes the blastopore closes and the mouth forms anew nearby (as in many flatworms and nematodes); the group is still protostome by the overall program and by molecular evidence.
• Modern molecular phylogeny splits protostomes into two giant clades — Ecdysozoa (moulting animals: arthropods, nematodes) and Lophotrochozoa (annelids, molluscs, flatworms and allies) — while confirming that deuterostomes form a single natural group.

For FSc and MDCAT purposes, the classical three differences are what matter. The molecular refinements are enrichment — impressive in a discussion answer, but the table above earns the marks.

Examiner tip

Four one-mark facts examiners repeat: (1) In deuterostomes the blastopore becomes the anus. (2) Spiral cleavage is determinate; radial cleavage is indeterminate (basis of identical twins). (3) Schizocoely = protostome; enterocoely = deuterostome. (4) Humans and starfish are both deuterostomes — closer to each other than either is to an insect.

Frequently Asked Questions

What is the main difference between protostomes and deuterostomes?

The main difference is the fate of the blastopore: in protostomes it becomes the mouth ("mouth first"), while in deuterostomes it becomes the anus and the mouth forms later ("mouth second"). They also differ in cleavage (spiral/determinate vs radial/indeterminate) and coelom formation (schizocoely vs enterocoely).

Are humans protostomes or deuterostomes?

Humans are deuterostomes. As chordates, we develop with radial, indeterminate cleavage, our blastopore becomes the anus, and our coelom forms by enterocoely. Starfish (echinoderms) are our fellow deuterostomes — closer relatives than any insect.

What is spiral cleavage, and which animals show it?

Spiral cleavage is an oblique cell-division pattern in which each new tier of cells sits in the grooves of the tier below, producing a spiral arrangement. It is determinate (each cell's fate is fixed early). It is characteristic of protostomes such as annelids (earthworm), molluscs (snail), and flatworms.

What is the difference between schizocoely and enterocoely?

Schizocoely is coelom formation by splitting of solid mesoderm blocks (characteristic of protostomes like annelids and arthropods). Enterocoely is coelom formation by outpocketing of the archenteron/gut wall (characteristic of deuterostomes like echinoderms and chordates).

Why is indeterminate cleavage important in humans?

Indeterminate (regulative) cleavage means early embryonic cells remain flexible — each can still form a complete embryo. This is the developmental basis of identical (monozygotic) twins: when the early human embryo splits, each part regulates into a whole baby. Protostomes, with determinate cleavage, cannot do this.

Which phyla are protostomes and which are deuterostomes?

Protostome phyla include Platyhelminthes, Nematoda, Annelida, Arthropoda, and Mollusca. Deuterostome phyla are only three: Echinodermata, Hemichordata, and Chordata. Sponges, cnidarians, and ctenophores belong to neither group.

Practice MCQs

Q1. In protostomes, the blastopore develops into the:

1. Anus
2. Mouth
3. Coelom
4. Archenteron

Answer: B — "Protostome" means mouth first: the blastopore becomes the mouth, while in deuterostomes it becomes the anus.

Q2. Spiral cleavage is characteristic of:

1. Deuterostomes
2. Protostomes
3. Cnidarians
4. Sponges

Answer: B — Protostomes show spiral, determinate cleavage; deuterostomes show radial, indeterminate cleavage.

Q3. Which of the following is a deuterostome?

1. Earthworm
2. Cockroach
3. Starfish
4. Snail

Answer: C — Starfish (Echinodermata) is a deuterostome; earthworm, cockroach, and snail are protostomes.

Q4. Schizocoelous coelom formation occurs in:

1. Deuterostomes
2. Protostomes
3. Cnidarians
4. Sponges

Answer: B — Protostomes form the coelom by splitting of mesoderm (schizocoely); deuterostomes use enterocoely.

Q5. Enterocoely is the formation of the coelom by:

1. Splitting of mesodermal bands
2. Outpocketing of the archenteron wall
3. Cavitation of the blastocoel
4. Invagination of the ectoderm

Answer: B — In enterocoely, pouches pinch off from the primitive gut wall and their cavities become the coelom.

Q6. Determinate cleavage is a feature of:

1. Deuterostomes
2. Protostomes
3. Echinoderms
4. Chordates

Answer: B — Protostome spiral cleavage is determinate (mosaic): each early cell's fate is fixed, so a removed blastomere cannot be replaced.

Q7. In deuterostome development, the mouth forms:

1. From the blastopore
2. As a secondary opening after the blastopore becomes the anus
3. By splitting of the mesoderm
4. From the archenteron directly

Answer: B — "Deuterostome" means mouth second: the blastopore becomes the anus and the mouth breaks through later.

Q8. Which animal is a protostome?

1. Sea urchin
2. Human
3. Earthworm
4. Balanoglossus

Answer: C — The earthworm (Annelida) is a protostome; sea urchin, human, and Balanoglossus are deuterostomes.

Q9. Radial cleavage is found in the embryos of:

1. Annelids and molluscs
2. Arthropods and nematodes
3. Echinoderms and chordates
4. Flatworms and roundworms

Answer: C — Echinoderms and chordates are deuterostomes with radial, indeterminate cleavage.

Q10. The mesoderm of protostomes arises from the:

1. Outpocketing of the archenteron
2. 4d mesentoblast cell near the blastopore
3. Invagination of the ectoderm
4. Cavitation of mesoglea

Answer: B — In protostomes the mesoderm comes from the 4d (mesentoblast) cell; in deuterostomes it buds off from the gut wall.

Q11. Which phylum is deuterostome?

1. Mollusca
2. Arthropoda
3. Echinodermata
4. Annelida

Answer: C — Echinodermata is one of only three deuterostome phyla (with Hemichordata and Chordata); the rest are protostomes.

Q12. Identical twins are possible in humans because human cleavage is:

1. Spiral and determinate
2. Radial and indeterminate
3. Spiral and indeterminate
4. Radial and determinate

Answer: B — Human (deuterostome) cleavage is radial and indeterminate, so separated early blastomeres can each regulate into a complete embryo.

Short Questions

Q1. What is the difference between protostomes and deuterostomes?

Protostomes ("mouth first") and deuterostomes ("mouth second") are the two great developmental branches of triploblastic bilateral animals. They differ in three classical embryonic characters. First, the fate of the blastopore: in protostomes the blastopore becomes the mouth, while in deuterostomes it becomes the anus and the mouth forms as a second opening. Second, the cleavage pattern: protostomes show spiral, determinate cleavage (each early cell's fate is fixed), whereas deuterostomes show radial, indeterminate cleavage (early cells remain flexible). Third, coelom formation: protostomes are schizocoelous (the coelom splits open within solid mesoderm), while deuterostomes are enterocoelous (the coelom pouches off from the gut wall).

The animal membership follows the development. Protostome phyla include Platyhelminthes, Nematoda, Annelida, Arthropoda, and Mollusca — earthworms, insects, snails, and roundworms. Deuterostome phyla are only three: Echinodermata (starfish), Hemichordata (Balanoglossus), and Chordata (fish through humans). A striking consequence is that a starfish is a closer relative of a human than an insect is, because both are deuterostomes.

For exam answers, present the three differences in a table — blastopore fate, cleavage, coelom formation — and add one example phylum per side. Mention the memory hooks: spiral = determinate = protostome, radial = indeterminate = deuterostome, and schizocoely ("split") = protostome versus enterocoely ("gut") = deuterostome. Examiners also love the identical-twins connection: only indeterminate (deuterostome) cleavage allows a split embryo to form two complete individuals.

Q2. What is the fate of the blastopore in protostomes and deuterostomes?

During gastrulation, the hollow blastula folds inward to form the archenteron (primitive gut), and the opening of this in-folding is the blastopore. In protostomes, the blastopore develops directly into the mouth of the adult; the anus forms later as a separate posterior opening (in a few protostomes the blastopore closes and both openings form anew, but the program remains protostome-like). In deuterostomes, the blastopore develops into the anus (or the anal region), and the mouth breaks through secondarily at the anterior end — hence "mouth second."

This single difference gives both groups their names: protostoma = first mouth, deuterostoma = second mouth. It is the most frequently examined of the three classical differences because it can be asked as a one-word fill-in: "In deuterostomes, the blastopore forms the ___" (anus). Students should also note that the blastopore-fate rule applies to the typical members; the classification of any animal ultimately rests on the whole developmental program plus molecular evidence, not on one character alone.

A useful way to remember it is the "pro = professional = first" hook: the protostome gets its mouth promptly (first), while the deuterostome’s mouth is delayed (second). Pair this with the coelom hook — schizocoely (split) for protostomes, enterocoely (gut) for deuterostomes — and the two most confused facts become hard to mix up.

Q3. Compare spiral and radial cleavage.

Spiral cleavage is the cell-division geometry of protostome embryos: successive tiers of cells divide at oblique angles, so each new tier settles into the grooves between the cells below, producing a spiral arrangement when viewed from the animal pole. Spiral cleavage is determinate (mosaic) — the developmental fate of every early blastomere is fixed, so destroying one cell deletes its contribution permanently. It is seen in annelids, molluscs, flatworms, and most other protostomes.

Radial cleavage is the geometry of deuterostome embryos: cells divide parallel or perpendicular to the egg axis, stacking in aligned tiers directly above one another like storeys of a building. Radial cleavage is indeterminate (regulative) — early blastomeres are not yet committed, and each can regulate to form a complete embryo if isolated. It is seen in echinoderms and chordates, including humans.

The determinate/indeterminate contrast has a famous human consequence: identical twins. Because human cleavage is indeterminate, an early embryo that splits in two can produce two complete babies — impossible under determinate protostome cleavage. In exams, always quote the paired triplets: spiral + determinate + protostome versus radial + indeterminate + deuterostome. Drawing the two cleavage diagrams side by side earns full marks in a "compare" question.

Q4. What is the difference between schizocoely and enterocoely?

Schizocoely ("split cavity") is the protostome method of coelom formation: solid blocks of mesoderm — budded from the 4d mesentoblast cell near the blastopore — take up positions on either side of the gut, and the coelomic cavity then opens as a split within each solid mass. Annelids (earthworm), arthropods (cockroach), and molluscs (snail) are textbook schizocoelomates. The memory hook is built into the word: schizo = split.

Enterocoely ("gut cavity") is the deuterostome method: the mesoderm arises as pouches pinched off from the wall of the archenteron (the primitive gut), and the hollow interiors of these pouches become the coelom. Echinoderms (starfish) and chordates (including humans) are textbook enterocoelomates. The hook here is equally direct: entero = gut — the cavity comes from the gut.

Examiners test this pair in two standard forms: "Name the type of coelom formation in protostomes/deuterostomes" and "Distinguish schizocoely from enterocoely." A complete answer names the method, the mechanism (splitting of mesoderm vs pouching of gut wall), the group (protostome vs deuterostome), and one example each (earthworm vs starfish). Note that acoelomate protostomes (flatworms) and pseudocoelomate protostomes (roundworms) never form a true coelom at all, so the schizocoely/enterocoely scheme applies only to truly coelomate members.

Q5. Why is a starfish considered closer to humans than an insect is?

Superficial resemblance suggests otherwise — a starfish looks nothing like a human, while an insect at least shares bilateral symmetry and jointed activity. But developmental and molecular evidence places the starfish (Echinodermata) and humans (Chordata) together in the deuterostome branch, while insects (Arthropoda) belong to the protostome branch. Both starfish and humans develop with radial, indeterminate cleavage, turn the blastopore into the anus, and form the coelom by enterocoely.

This relationship is confirmed by DNA-based phylogeny, which groups echinoderms with hemichordates and chordates in a single deuterostome clade, separate from all protostomes. Shared deuterostome features beyond embryology include the pattern of gill-slit-like pharyngeal structures (in hemichordates and chordates) and distinctive larval forms. The lesson is a central principle of modern zoology: early development and molecules reveal truer relationships than adult appearance.

This fact is an examiner favorite precisely because it is counterintuitive — it tests whether the student has memorized the deuterostome member list (Echinodermata, Hemichordata, Chordata) rather than reasoning from looks. In a discussion answer, state the claim, cite the three shared deuterostome developmental characters, and conclude with the principle that classification follows development and DNA, not external resemblance.

Q6. Name the protostome and deuterostome phyla with one example each.

The protostome phyla are Platyhelminthes (flatworms — e.g., Planaria), Nematoda (roundworms — e.g., Ascaris), Annelida (segmented worms — e.g., earthworm), Arthropoda (jointed-legged animals — e.g., cockroach), and Mollusca (soft-bodied animals — e.g., garden snail), plus smaller phyla such as Rotifera. All share spiral determinate cleavage and the mouth-first developmental program.

The deuterostome phyla are only three: Echinodermata (spiny-skinned animals — e.g., starfish), Hemichordata (acorn worms — e.g., Balanoglossus), and Chordata (notochord animals — e.g., fish, frog, human). All share radial indeterminate cleavage, blastopore-to-anus development, and enterocoely.

Two fine points complete the answer. First, sponges, cnidarians, and ctenophores are neither — they branched off before the protostome-deuterostome split. Second, modern molecular work divides protostomes into Ecdysozoa (moulting animals: arthropods, nematodes) and Lophotrochozoa (annelids, molluscs, flatworms), a useful enrichment line for high-scoring answers. Present the phyla as two labeled lists with examples, and the question is fully answered.

Conclusion: Two Ways to Build an Animal

The protostome vs deuterostome split shows that there are two fundamentally different recipes for building a bilateral animal. Protostomes — the earthworms, insects, and snails that dominate animal diversity — build mouth-first, with spiral determinate cleavage and a coelom split from solid mesoderm. Deuterostomes — starfish, acorn worms, and the entire chordate line leading to humans — build anus-first, with radial indeterminate cleavage and a coelom pouched from the gut. Three embryonic characters, memorized as paired triplets, unlock the whole topic: blastopore fate, cleavage pattern, and coelom formation. And the deepest lesson is a humbling one for our species: by development and by DNA, we are closer to a starfish than to a cockroach.