The comparison scores every group at once. This page is for the moment you already have a pair of names in mind — mayfly or stonefly, scud or sowbug, shad or alewife — and want the structures that keep them apart, in one look. Nothing here is a current hatch.
The differences, and nothing else
Best separating check: Leg-like flaps down both sidesClose look It separates Marine worms from Aquatic worms cleanly, and every pair of groups still in play differs on it (look closely with it still).
What to look at
Marine worms
Aquatic worms
Head everts a proboscis with dark jawsdecisiveIn hand
Yes
No
Leg-like flaps down both sidesdecisiveClose look
Yes
No
School or bait cluster, not a single insectWatch it
Not diagnostic
No
Reported size
14–900 mm
1–150 mm
Water it is recorded indecisive
inshore, nearshore
lake margin, river, stream, reservoir
How it moves
swarming adults swim at the surface; males swim faster in a gyrating motion, females circling with males close behind; like many bristle worms, breaks apart easily when handled and regrows the lost part
stretching and pulling like an earthworm; waving in the water while anchored in the sediment
The same for both: life stages, seasonality.
Rows come from the reviewed diagnostic weights the comparison scores with. “Decisive” means one answer puts them at least four points apart — the carrier's own rule for a structure that separates a pair.
The two entries, side by side
Polychaeta
Marine worms
The short red one swimming at the surface is the same animal as the long brown one in the mud.
First check
Look for the row of leg-like flaps down both sides. Then get the proboscis out if you can: two jaws is a clam worm or sandworm, four is a bloodworm.
What it is
Start with the fact that reorganises everything else. When a clam worm breeds it does not send out a different species — it rebuilds itself. The body shortens from up to 170 millimetres down to somewhere between 14 and 75, the eyes enlarge, the middle section grows flattened paddle-like parapodia, and the males turn bright red with a white tail. Then the breeding worms leave the bottom, often in immense numbers, and swim at the surface, males chasing females in a gyrating circle. A short, bright, big-eyed worm swimming in open water is not a new animal. It is the same worm, dressed for one night's work.
Stages
Bottom form and breeding form (On the bottom: clam worm up to about 170 mm; sandworm usually 200–400 mm, occasionally near a metre. The clam worm's swarming form is 14–75 mm.)
Water
inshore, surf, nearshore
Separates from Aquatic worms
No flaps down the sides at all and no proboscis points to a freshwater aquatic worm rather than a marine one.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
None mapped yet.
Limits
“Cinder worm” is used by no agency, Sea Grant or university source consulted; it appears only in angling media. The Smithsonian does establish that Nereis limbata and Alitta succinea are the same animal, which is the strongest link available, but nothing consulted says the animal anglers call a cinder worm is that species.
Segmented, round, no legs, no hard head — then look for bristle bundles and check both ends for suckers.
What it is
A soft segmented tube, round in cross section, with no legs and no hardened head. Bundles of fine bristles run along the segments and are the character that separates it from a leech.
Stages
Adult (1–150 mm across the group, most 1–30 mm; tubificids to about 25 mm)
Water
lake-margin, river, stream, reservoir
Separates from Marine worms
This entry does not name Marine worms as a look-alike. That is a gap in the prose, not proof they cannot be confused.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
None mapped yet.
Limits
The regional guide gives 1–30 mm for most aquatic worms and up to 150 mm for the largest; treat the upper figure as the extreme, not the norm.