Cnidarians crossword puzzle with educational clues about marine organisms.

Let’s solve this crossword puzzle step by step. We’ll go clue by clue, using the definitions and word lengths (given in parentheses) to find the right answers.

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Across Clues:

1. Cnidarians such as box jellyfish (8)
→ Box jellyfish are a type of cnidarian. The group that includes them is “Scyphozoa” — but that’s 9 letters. Wait — maybe “Medusae”? That’s 7. Hmm. Let’s think: “Box jellyfish” belong to class Cubozoa — too short. Maybe the answer is “Jellyfish”? But that’s not specific. Wait — perhaps it’s “Hydrozoa”? No. Let’s hold on.

Actually, looking at common crossword answers for “cnidarians such as box jellyfish”, the likely answer is “Medusoid”? Too long. Or maybe “Polyp”? Too short.

Wait — let’s look at Down clues first — sometimes they help fill in letters.

But let’s try again: Box jellyfish are in class Cubozoa — 7 letters. Not 8.

Another idea: “Cnidaria” itself is 8 letters — but that’s the whole phylum.

Wait — perhaps the answer is “Aurelia”? That’s a genus of jellyfish — 7 letters.

Hold on — maybe it’s “Scyphozoan”? 10 letters.

This is tricky. Let’s skip and come back.

2. An animal that has radial symmetry (6)
→ Cnidarians have radial symmetry. A simple one-word answer? “Jelly” — no. “Starfish” — 8 letters. “Sea anemone” — too long.

Wait — maybe “Medusa”? 6 letters! Yes — medusa is the free-swimming form of many cnidarians, and they have radial symmetry. So Medusa fits.

So Across 2 = MEDUSA

3. A passive stage fed by water currents (5)
→ In cnidarians, the polyp stage is often sessile (attached), but some drift. Wait — “passive stage fed by water currents” — could be “polyp”? 5 letters? POLYP is 5 — yes!

Polyps can be passive and filter feed from water currents. So Across 3 = POLYP

4. A reproductive structure (6)
→ In cnidarians, gonads produce gametes. “Gonad” is 5. “Gamete” is 6 — yes! Gametes are reproductive cells. So Across 4 = GAMETE

5. A stinging cell (9)
→ This is classic: NEMATOCYST — 10 letters? Wait — NEMATOCYST is 10. But clue says 9.

Wait — maybe “CNIDOCYTE”? That’s 9 letters! Yes — cnidocyte is the cell that contains the nematocyst. Sometimes used interchangeably, but technically cnidocyte is the cell, nematocyst is the organelle inside.

In many textbooks, “stinging cell” refers to cnidocyte. So Across 5 = CNIDOCYTE

6. The type of symmetry exhibited by most cnidarians (6)
→ Radial symmetry. “RADIAL” is 6 letters. Perfect. Across 6 = RADIAL

7. An animal with stinging cells (9)
→ Again, cnidarian. “CNIDARIAN” is 9 letters. Yes! Across 7 = CNIDARIAN

8. A cnidarian such as a jellyfish (8)
→ Back to clue 1. Now we know more. If Across 7 is CNIDARIAN, then Across 8 might be “MEDUSOID”? 8 letters? MEDUSOID is 8 — yes! Medusoid refers to the jellyfish-like form.

Or maybe “SCYPHOZOAN”? 10 letters.

Wait — “JELLYFISH” is 9.

Another possibility: “HYDROMEDUSA”? Too long.

Perhaps “AURELIA”? 7.

Wait — let’s check Down clues to get intersecting letters.

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Down Clues:

1. A sleeping cell or embryo (5)
→ In biology, a dormant stage — like a cyst? Or spore? “SPORE” is 5 — possible. “CYST” is 4. “EGG” is 3. “ZYGOTE” is 6.

Wait — in cnidarians, some have a resting stage called a “planula larva” — too long.

Maybe “POLYP”? Already used.

Another idea: “TENTACLE”? No.

Wait — perhaps “SPONGE”? No.

Think: “sleeping cell or embryo” — could be “OVUM”? 4 letters.

Wait — “BLASTULA”? 8.

Hmm. Maybe “GERM”? 4.

Not fitting. Let’s look at other down clues.

2. To grow larger (6)
→ GROWTH? 6 letters. Or INCREASE? 8. EXPAND? 6 — possible. DEVELOP? 7.

“GROWTH” is a noun — clue says “to grow larger” — verb. So “INFLATE”? No.

“ENLARGE”? 7.

“SWELL”? 5.

Wait — “PROLIFERATE”? Too long.

Maybe “MULTIPLY”? 8.

Another idea: “BLOOM” — 5.

Wait — perhaps “THRIVE”? 6 — but not exactly “grow larger”.

Maybe “EXPAND” — 6 letters. Let’s tentatively put EXPAND.

But let’s see if we can get better matches.

3. A gelatinous substance (9)
→ Mesoglea! That’s the jelly-like layer in cnidarians. MESOGLEA is 8 letters. Wait — M-E-S-O-G-L-E-A → 8.

Clue says 9. Hmm.

“COLLAGEN”? 8.

“GELATIN”? 7.

“SUBSTANCE” — too vague.

Wait — maybe “MESOGLOEA”? Alternative spelling? Still 8.

Perhaps “JELLYLIKE”? 9 — but not standard.

Another thought: “HYALINE”? 7.

Wait — perhaps “TRANSPARENT”? 11.

No. Let’s reconsider mesoglea — maybe the puzzle accepts 8-letter answer even if clue says 9? Unlikely.

Wait — maybe “ECTODERM”? 8.

I’m stuck here.

4. Empty space (4)
→ SPACE? 5. VOID? 4 — yes! Or HOLE? 4. GAP? 3.

VOID is good. DOWN 4 = VOID

5. A shape with circular holes in projecting spines (like sea urchin) (7)
→ Sea urchins have test with pores. But cnidarians? Maybe referring to something else.

Wait — “spines” — cnidarians don’t typically have spines. Sea urchins are echinoderms.

Perhaps this is misdirection? Or maybe it’s about the structure of a colony?

Another idea: “PORIFERA”? 8 — sponges.

Wait — “RADIOLARIA”? 10.

Perhaps “TEST”? 4.

Not helping.

6. A cnidarian with several arms (8)
→ Like a hydra? HYDRA is 5. Anemone? 7. Jellyfish? 9.

“OCTOPUS”? 7 — but not cnidarian.

Wait — “CORAL”? 5.

“SEA ANEMONE” — 10.

Perhaps “ACTINIAN”? 8 — actinian is another name for sea anemone. YES! ACTINIAN is 8 letters. Down 6 = ACTINIAN

7. A cnidarian with a hard skeleton (8)
→ Coral! CORAL is 5. “HARD CORAL”? 9.

“SCLERACTINIA”? Too long.

“CALCAREOUS”? 10.

Wait — “CORALLITE”? 9.

Perhaps “REEF”? 4.

Another idea: “STONY CORAL” — too long.

Wait — “ANTHOZOA”? 8 — Anthozoa is the class that includes corals and sea anemones, and many have hard skeletons. Yes! ANTHOZOA is 8 letters. Down 7 = ANTHOZOA

8. A cnidarian such as a coral (6)
→ POLYP? Already used. ANEMONE? 7. CORAL? 5.

“ZOANTHID”? 8.

Wait — “CORAL” is 5 — not 6.

“ANTHOZOAN”? 9.

Perhaps “HYDROID”? 7.

Wait — “MEDUSA”? Already used.

Another idea: “PLANULA”? 7 — larval stage.

Not fitting.

Wait — maybe “POLYPOID”? 8.

I’m stuck.

Let’s go back to Across 1: “Cnidarians such as box jellyfish (8)”

Box jellyfish are in class Cubozoa — 7 letters.

But perhaps the answer is “CUBOZOAN”? 8 letters! C-U-B-O-Z-O-A-N → 8 letters. Yes! That must be it.

So Across 1 = CUBOZOAN

Then Across 8: “A cnidarian such as a jellyfish (8)” — now we can use intersecting letters.

If Across 1 is CUBOZOAN, and it crosses with Down 1, etc.

Also, Across 8 might be “SCYPHOZOAN” — 10 letters — no.

“HYDROZOAN”? 9.

Wait — “MEDUSOID” is 8 — and it means jellyfish-like. So Across 8 = MEDUSOID

Now let’s fill in what we have:

Across:
1. CUBOZOAN
2. MEDUSA
3. POLYP
4. GAMETE
5. CNIDOCYTE
6. RADIAL
7. CNIDARIAN
8. MEDUSOID

Down:
1. ? (5 letters) — intersects with CUBOZOAN at position 1: C _ _ _ _ _ _ _
So Down 1 starts with C.
“A sleeping cell or embryo (5)” — starting with C.
Possible: CYST? 4. CELL? 4. CONCH? No.
“COCOON”? 6.
“CRYPT”? 5 — but not biological.
“CYSTE”? Not English.
Wait — “SPOROCYST”? Too long.
Another idea: “ZOOID”? 5 — but not sleeping.
Perhaps “DIAPAUSE”? 8.
I’m stuck.

Wait — maybe “POLYP” is already used, but Down 1 could be “POLYP” if it fits? But Across 3 is POLYP, so if Down 1 intersects at letter 3 of Across 1, which is B (CUBOZOAN: positions 1=C,2=U,3=B,4=O,5=Z,6=O,7=A,8=N)

So Down 1 starts at row 1, column 1 — which is C.

So Down 1: 5 letters, starts with C.

“A sleeping cell or embryo” — in botany or zoology, a “cyst” is a dormant stage, but 4 letters.

“Cryptobiosis” — too long.

Perhaps “CONCHA”? No.

Another thought: in some contexts, “egg” is called “ovum”, but not 5 letters.

Wait — “BLASTOCYST”? 10.

Perhaps the answer is “SPORE” — but doesn't start with C.

Unless... maybe I have Across 1 wrong.

Alternative for Across 1: “JELLYFISH” is 9 — no.

“BOX JELLY” — 8 letters? BOXJELLY — not standard.

Perhaps “CUBOZOA” — 7 letters.

I think CUBOZOAN is correct.

Let’s assume Down 1 is “CYST” — but 4 letters, clue says 5.

Wait — maybe “CYSTS” — plural? 5 letters. And "a sleeping cell" could be referred to as a cyst in some contexts. For example, in protozoans, but cnidarians? Some have cyst stages.

Perhaps acceptable. So Down 1 = CYSTS

Then Down 2: “To grow larger (6)” — intersects with Across 2: MEDUSA — position 2 is E.

So Down 2 starts at row 2, column 2 — which is E.

6 letters, starts with E.

“To grow larger” — ENLARGE? 7. EXPAND? 6 — but starts with E? EXPAND starts with E — yes! E-X-P-A-N-D — 6 letters. And it means to grow larger. So Down 2 = EXPAND

Down 3: “A gelatinous substance (9)” — intersects with Across 3: POLYP — position 3 is L.

So Down 3 starts at row 3, column 3 — which is L.

9 letters, starts with L.

Gelatinous substance in cnidarians — mesoglea, but doesn't start with L.

“LIMPID”? No.

“LIQUID”? 6.

“LOBULE”? 6.

Perhaps “MESOGLEA” is intended, but doesn't fit.

Another idea: “COLLOID”? 7.

Wait — “LAMINA”? 6.

Not working.

Perhaps “JELLY” — 5.

I recall that in some puzzles, “mesoglea” is accepted as 8, but clue says 9.

Wait — maybe “ENDODERM”? 8.

Perhaps the answer is “HYALOPHANE”? Too obscure.

Another thought: “GELATINOUS” — 10.

Let’s skip.

Down 4: “Empty space (4)” — we said VOID. Intersects with Across 4: GAMETE — position 4 is E.

So Down 4 starts at row 4, column 4 — which is E.

4 letters, starts with E.

“Empty space” — EMPTY? 5. VOID? 4 — but starts with V.

“EVE”? No.

“EAR”? No.

“EDGE”? 4 — but not empty space.

“EXpanse”? 7.

Perhaps “NULL”? 4 — starts with N.

Not matching.

If Across 4 is GAMETE, then column 4 is M? Let's map the grid.

Assume the crossword grid is standard. From the image description, it's a typical crossword.

Perhaps I need to list all answers based on common knowledge.

Let me search my memory for standard cnidarian crossword answers.

Common answers:

- Stinging cell: CNIDOCYTE or NEMATOCYST — but NEMATOCYST is 10, CNIDOCYTE is 9 — clue 5 across is 9, so CNIDOCYTE.

- Symmetry: RADIAL

- Phylum: CNIDARIA — but clue 7 across is 9, CNIDARIAN is 9.

- Class for box jellyfish: CUBOZOA — 7, but clue 1 across is 8, so CUBOZOAN.

- Jellyfish form: MEDUSA

- Polyp stage: POLYP

- Reproductive cell: GAMETE

- Gelatinous layer: MESOGLEA — 8 letters, but clue down 3 is 9 — perhaps it's "MESOGLOEA" with extra e? Still 8.

Wait — maybe "THEMESOGLEA"? No.

Another idea: "JELLYFISH" for down 3? 9 letters — J-E-L-L-Y-F-I-S-H — 9 letters. And it's gelatinous. But "jellyfish" is an animal, not a substance.

The substance is mesoglea.

Perhaps the puzzle has a mistake, or I need to accept MESOGLEA as 8 for a 9-letter clue — unlikely.

Let's look at Down 5: "A shape with circular holes in projecting spines (like sea urchin) (7)"

Sea urchins have a test with ambulacral pores. But for cnidarians? Perhaps it's referring to the structure of a coral colony or something.

"PORIFERA" is sponges, 8 letters.

"RADIOLARIAN" — 11.

"FORAMINIFERA" — 12.

Perhaps "TEST" — 4.

Another idea: "SPICULE" — 7 letters! Spicules are skeletal elements in sponges and some other animals, and they can have holes. But cnidarians don't typically have spicules; sponges do.

The clue says "like sea urchin" — sea urchins have a test made of plates with pores.

Perhaps "PLATELET"? 8.

"OSCULUM"? 7 — but that's the large opening in sponges.

Osculum is 7 letters, and it's a hole, but not with spines.

Perhaps "POROUS" — 6.

I think "SPICULE" might be intended, even though it's more sponge-related.

Down 5 = SPICULE

Down 6: "A cnidarian with several arms (8)" — we said ACTINIAN — sea anemone, which has many arms/tentacles. 8 letters. Good.

Down 7: "A cnidarian with a hard skeleton (8)" — ANTHOZOA — the class that includes corals, which have hard skeletons. 8 letters. Good.

Down 8: "A cnidarian such as a coral (6)" — intersects with Across 8: MEDUSOID — position 8 is D.

So Down 8 starts at row 8, column 8 — which is D.

6 letters, starts with D.

"Coral" is 5, so perhaps "DIPLOPORA"? Too long.

"DEAD CORAL"? No.

"DRIFTING"? No.

Another idea: "HYDROID" — 7.

"POLYPOID" — 8.

Perhaps "CORAL" is accepted as 6? No.

Wait — "ANTHOZOAN" is 9.

Perhaps "ZOANTHID" — 8.

I recall that "coral" can be referred to as "madrepore" — 9 letters.

Not helping.

Perhaps "REEF" — 4.

Let's consider that Down 8 might be "POLYP" — but 5 letters, and already used.

Another thought: "MEDUSA" is 6 letters — but already used for Across 2.

And it starts with M, not D.

Unless the grid is different.

Perhaps for Down 8, the answer is "CORALS" — 6 letters! C-O-R-A-L-S — 6 letters. And "a cnidarian such as a coral" — corals are cnidarians, so "corals" works. And it starts with C, but we need it to start with D if it intersects with MEDUSOID at position 8.

MEDUSOID: positions 1=M,2=E,3=D,4=U,5=S,6=O,7=I,8=D

So if Down 8 is at column 8, row 8, then it starts with D.

"CORALS" starts with C, not D.

"DEADLY"? No.

"DENDRITE"? 8.

Perhaps "DIPLOZOA"? Not real.

I think there might be a mistake in my assumption.

Let's try to fill the grid mentally.

Assume the crossword is 8x8 or similar.

From the clues, Across 1 is 8 letters, Across 2 is 6, etc.

Perhaps Down 1 is "SPORO" — 5 letters, but not standard.

Another idea for Down 1: "EGG" is 3, "OVUM" is 4, "ZYGOTE" is 6.

"BLASTULA" is 8.

Perhaps "DIAPAUSAL" — 9.

I recall that in some biology contexts, "cryptobiotic state" but not 5 letters.

Let's guess Down 1 = "CYSTS" as before.

Then for Down 3: "A gelatinous substance (9)" — perhaps "MESOGLEAL" — not a word.

"JELLYFISH" is 9, and although it's an animal, it's gelatinous, so maybe accepted. Starts with J, but if it intersects with POLYP at position 3, which is L, so must start with L.

POLYP: P-O-L-Y-P — so position 3 is L.

So Down 3 starts with L.

9 letters, starts with L, gelatinous substance.

"LIQUIDITY"? 9 — but not a substance.

"LOBSTER"? No.

"LIMPIDITY"? 9 — clarity, not substance.

Perhaps "LAMINARIN"? A polysaccharide, 9 letters, but not gelatinous in cnidarians.

I think I need to accept that "mesoglea" is the intended answer, and perhaps the clue has a typo, or in some spellings it's 9 letters.

Upon second thought, "mesoglea" is sometimes spelled "mesogloea" — still 8 letters.

M-E-S-O-G-L-O-E-A — 9 letters! Yes! If you spell it with 'oe' instead of 'e', it's 9 letters.

In British English, "oesophagus" etc., so "mesogloea" is 9 letters.

So Down 3 = MESOGLOEA

Perfect.

Then Down 4: "Empty space (4)" — intersects with GAMETE at position 4.

GAMETE: G-A-M-E-T-E — so position 4 is E.

Down 4 starts with E, 4 letters.

"Empty space" — "EVE"? No.

"EAR"? No.

"EDGE"? 4 — but not empty space.

"EXpanse" — 7.

"NULL" — starts with N.

"VOID" — starts with V.

Perhaps "EFT" — 3.

Another idea: "AIR" — 3.

"SPACE" — 5.

Perhaps "HOLE" — 4, but starts with H.

Unless the intersection is not at position 4.

Let's define the grid.

Assume the crossword has Across clues numbered, and Down clues numbered, and they intersect.

For example, Across 1: 8 letters, so occupies columns 1-8, row 1.

Down 1: 5 letters, so rows 1-5, column 1.

So at (1,1) = C (from CUBOZOAN)

Down 1: 5 letters, starts with C.

"A sleeping cell or embryo" — let's say "CYSTS" — C-Y-S-T-S — 5 letters. Acceptable.

Then Across 2: 6 letters, probably starts at row 2, column 1 or later.

Typically, Across 2 might start at row 2, column 1, but if Down 1 is in column 1, rows 1-5, then Across 2 might start at row 2, column 2 or something.

To simplify, let's list all answers based on best fit.

Final decision:

Across:
1. CUBOZOAN
2. MEDUSA
3. POLYP
4. GAMETE
5. CNIDOCYTE
6. RADIAL
7. CNIDARIAN
8. MEDUSOID

Down:
1. CYSTS (assuming "cysts" as dormant stages)
2. EXPAND
3. MESOGLOEA (9 letters, gelatinous substance)
4. VOID — but must start with E if intersects with GAMETE at position 4. GAMETE is G-A-M-E-T-E, so if Down 4 is at column 4, and Across 4 is in row 4, then at (4,4) = E, so Down 4 starts with E.

4 letters, starts with E, "empty space".

Possible words: "EVE", "EAR", "EGO", "EFT", "ELK", "EMU", "END", "EON", "ERA", "ERR", "EST", "ETA", "EVE", "EWE", "EYE" — none mean "empty space".

"EMPTY" is 5.

Perhaps "EVEN" — 4, but not empty space.

Another idea: "NULL" is 4, but starts with N.

Perhaps the answer is "ZERO" — 4, starts with Z.

Not matching.

Perhaps "AIR" is 3.

I think there might be a mistake. Let's consider that Across 4 is not GAMETE.

Clue 4 across: "A reproductive structure (6)" — could be "OVARY" — 5, "TESTIS" — 6, but not general.

"GONAD" — 5.

"SPERM" — 5.

"EGG" — 3.

"ZYGOTE" — 6 — and it's a reproductive structure (fertilized egg). ZYGOTE is 6 letters.

And it starts with Z.

Then if Down 4 intersects at position 4 of Across 4, which would be O (Z-Y-G-O-T-E), so position 4 is O.

Then Down 4 starts with O, 4 letters, "empty space".

"OPEN" — 4, but not empty space.

"ONLY" — 4.

"OVER" — 4.

"OWLS" — 4.

Not good.

"VOID" starts with V.

Perhaps "HOLE" — 4, starts with H.

I think for Down 4, the answer is "VOID", and the intersection is not at that position, or the grid is arranged differently.

Perhaps "SPACE" is 5, but clue says 4.

Another possibility: "GAP" — 3.

I recall that in some crosswords, "nil" is used for zero or empty, but 3 letters.

Let's assume Down 4 = "VOID" and move on.

Down 5: "A shape with circular holes in projecting spines (like sea urchin) (7)" — we said SPICULE — 7 letters. Spicules can have holes, and are found in some animals. Although more characteristic of sponges, it might be accepted.

Down 6: ACTINIAN
Down 7: ANTHOZOA
Down 8: "A cnidarian such as a coral (6)" — and it starts with D if intersects with MEDUSOID at position 8.

MEDUSOID ends with D, so Down 8 starts with D.

6 letters, starts with D, cnidarian like coral.

"DEADLY"? No.

"DENDRITE"? 8.

"DIPLOZOA"? Not real.

"DRIFTING"? No.

Perhaps "CORAL" is 5, so "CORALS" is 6, but starts with C.

Unless the last letter of MEDUSOID is not the start of Down 8.

Perhaps Down 8 is at a different position.

Maybe "HYDRA" is 5, "ANEMONE" is 7.

Another idea: "POLYP" is 5, "MEDUSA" is 6 — but already used.

Perhaps "ZOANTHID" is 8.

I think the only reasonable answer is "CORALS", and assume the grid allows it, or perhaps the answer is "ANTHOZOAN" but 9 letters.

Let's calculate: if Down 8 is 6 letters, and starts with D, perhaps "DIPLOPOR" — not a word.

Perhaps "DENDRIFORM" — 10.

I give up on that for now.

For Down 2: "To grow larger (6)" — we have EXPAND, which is good.

For Down 1: CYSTS

For Down 3: MESOGLOEA

For Down 4: let's say "VOID" and assume the intersection is not conflicting, or perhaps it's "EVEN" but not accurate.

Another possibility for Down 4: "EFT" is 3, "ERA" is 3.

Perhaps "EMPTY" is 5, but clue says 4.

Let's look online or recall that in some puzzles, "nil" is used, but 3 letters.

Perhaps "ZERO" — 4, and "zero space" means empty, but starts with Z.

If Across 4 is ZYGOTE, then position 4 is O, so Down 4 starts with O.

"OPEN" — 4, and "open space" means empty space. Yes! "Open" can mean empty or unoccupied.

So Down 4 = OPEN

Perfect.

Then Across 4 = ZYGOTE (6 letters, reproductive structure)

But earlier I had GAMETE, which is also correct, but ZYGOTE fits better with the intersection.

GAMETE is a reproductive cell, ZYGOTE is a reproductive structure (the cell formed by fusion).

Both are valid, but ZYGOTE allows Down 4 to be OPEN.

So let's change Across 4 to ZYGOTE.

Then Down 4 = OPEN

Now Down 8: "A cnidarian such as a coral (6)" — starts with D (from MEDUSOID ending with D).

6 letters, starts with D.

What cnidarian starts with D? "Dendrophyllia" — too long.

"Desmophyllum" — 12.

Perhaps "DIPLOZOA" — not real.

Another idea: "DEAD CORAL" — not 6.

Perhaps "DRIFTING" — no.

I recall that "coral" can be "madrepora" — 9.

Perhaps the answer is "HYDROID" — 7.

Let's consider that "coral" is sometimes called "anthozoan", but 9 letters.

Perhaps "POLYPOID" — 8.

I think there might be a mistake, or perhaps "MEDUSA" is used, but it's 6 letters and starts with M.

Unless the last letter of MEDUSOID is not the start of Down 8.

Perhaps Down 8 is at column 1 or something.

To resolve, let's assume that for Down 8, the answer is "CORALS", and the grid has it starting at a different position, or perhaps it's "ANTHOZOAN" but 9 letters.

Another possibility: "SEA FAN" — 6 letters? S-E-A-F-A-N — 6, but "sea fan" is a type of coral, and it's two words.

Usually crosswords allow multi-word, but clue says "a cnidarian", so "seafan" as one word? 6 letters.

S-E-A-F-A-N — 6 letters. And it starts with S, not D.

Not matching.

Perhaps "DENDRITE" — 8.

I think I'll go with "CORALS" for Down 8, and assume the intersection is not at the end of MEDUSOID, or perhaps MEDUSOID is not correct.

For Across 8: "A cnidarian such as a jellyfish (8)" — we have MEDUSOID, but perhaps it's "SCYPHOZOAN" — 10, no.

"HYDROZOAN" — 9.

"JELLYFISH" — 9.

"BOX JELLY" — 8 letters if combined: BOXJELLY — not standard.

Perhaps "CUBOZOAN" is for Across 1, and Across 8 is "MEDUSAE" — 7 letters.

MEDUSAE is 7, clue says 8.

"MEDUSANS" — 8 letters! M-E-D-U-S-A-N-S — 8 letters. And it means relating to medusae, or medusans are jellyfish-like cnidarians.

So Across 8 = MEDUSANS

Then it ends with S, so Down 8 starts with S.

6 letters, starts with S, "a cnidarian such as a coral".

"SPONGE" — 6, but not cnidarian.

"SEA ANEMONE" — 10.

"SCLERACTINIA" — 12.

"STONY CORAL" — 10.

"SOFT CORAL" — 9.

"SPICULE" — 7.

"SPOROCYST" — 9.

Perhaps "SPONGILLA" — 9.

Another idea: "SIPHONOPHORE" — 12.

Perhaps "SERTULARIA" — 10.

I think "CORALS" is still the best, but starts with C.

Unless "S" is for "SEA URCHIN" — not cnidarian.

Perhaps "SALPA" — 5.

I recall that "coral" can be "gorgonian" — 9.

Let's try "SCLERACTIN" — 10.

Perhaps the answer is "ANTHOZOAN" but 9 letters.

For Down 8, if it's 6 letters, starts with S, perhaps "SPONGE" is accepted, but it's not cnidarian.

The clue says "a cnidarian such as a coral", so it must be a cnidarian.

"SEA PEN" — 6 letters? S-E-A-P-E-N — 6, and sea pens are cnidarians, related to corals.

Yes! Sea pens are in the same class as corals, Anthozoa.

So "SEAPEN" or "SEA PEN" — as one word "SEAPEN" is 6 letters.

S-E-A-P-E-N — 6 letters. And it starts with S.

Perfect.

So Down 8 = SEAPEN

Great.

Now Down 5: "A shape with circular holes in projecting spines (like sea urchin) (7)" — we have SPICULE, but spicules are not typically with circular holes; sea urchins have a test with pores.

"PORIFERA" is 8.

"RADIOLARIAN" — 11.

"FORAMINIFERA" — 12.

"TEST" — 4.

"PLATE" — 5.

"OSCULUM" — 7, and it's a large pore in sponges, but not with spines.

"SPICULE" is the best available.

Perhaps "POROUS" — 6.

I think "SPICULE" is acceptable.

So Down 5 = SPICULE

Now Down 1: "A sleeping cell or embryo (5)" — we have CYSTS, which is plausible.

Some sources use "cyst" for dormant stages in various organisms.

So we'll keep it.

Down 2: EXPAND

Down 3: MESOGLOEA

Down 4: OPEN

Down 6: ACTINIAN

Down 7: ANTHOZOA

Down 8: SEAPEN

Now let's verify all:

Across:
1. CUBOZOAN - cnidarians such as box jellyfish (8) ✓
2. MEDUSA - an animal that has radial symmetry (6) ✓ (medusa is a cnidarian with radial symmetry)
3. POLYP - a passive stage fed by water currents (5) ✓
4. ZYGOTE - a reproductive structure (6) ✓ (fertilized egg)
5. CNIDOCYTE - a stinging cell (9) ✓
6. RADIAL - the type of symmetry exhibited by most cnidarians (6) ✓
7. CNIDARIAN - an animal with stinging cells (9) ✓
8. MEDUSANS - a cnidarian such as a jellyfish (8) ✓ (medusans are jellyfish-like cnidarians)

Down:
1. CYSTS - a sleeping cell or embryo (5) ✓ (dormant stages)
2. EXPAND - to grow larger (6) ✓
3. MESOGLOEA - a gelatinous substance (9) ✓ (alternative spelling of mesoglea)
4. OPEN - empty space (4) ✓ (as in open space)
5. SPICULE - a shape with circular holes in projecting spines (7) ✓ (spicules can have holes, though more sponge-related, but accepted)
6. ACTINIAN - a cnidarian with several arms (8) ✓ (sea anemone)
7. ANTHOZOA - a cnidarian with a hard skeleton (8) ✓ (class including corals)
8. SEAPEN - a cnidarian such as a coral (6) ✓ (sea pen is a type of coral relative)

All seem reasonable.

So the final answers are:

Across:
1. CUBOZOAN
2. MEDUSA
3. POLYP
4. ZYGOTE
5. CNIDOCYTE
6. RADIAL
7. CNIDARIAN
8. MEDUSANS

Down:
1. CYSTS
2. EXPAND
3. MESOGLOEA
4. OPEN
5. SPICULE
6. ACTINIAN
7. ANTHOZOA
8. SEAPEN

Now, since the user asked to solve the problem, and the task is to fill the crossword, the final answer should be the completed grid or the list of answers.

But typically, for such requests, we provide the answers in order.

Since the instruction is to provide only the final correct answer after reasoning, and the answer is the set of words, I'll list them as per the clues.

But to match the format, perhaps just state that the crossword is solved with these words.

Since the user didn't specify how to present, I'll provide the answers in a clear list.

Final Answer:
Across: 1. CUBOZOAN, 2. MEDUSA, 3. POLYP, 4. ZYGOTE, 5. CNIDOCYTE, 6. RADIAL, 7. CNIDARIAN, 8. MEDUSANS
Down: 1. CYSTS, 2. EXPAND, 3. MESOGLOEA, 4. OPEN, 5. SPICULE, 6. ACTINIAN, 7. ANTHOZOA, 8. SEAPEN