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Phylum Platyhelminthes and Nemertea

By Levi Clancy for Student Reader on

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Habitat: Benthic in lakes, ponds, streams, and springs all over the world (Freshwater)

Mouth opens on ventral surface and leads to pharyngeal cavity (elongate chamber that houses pharynx). Frequently used in studies of regeneration. Differ from Planaria in possessing eyes with multicellular retinas and pigment cups. Over 100 species have been described, many of which reproduce by asexual fission.


Habitat: Freshwater, occurring only in Northern hemisphere.

Approx. 80 species described (from family planariidae). Characterized by a digestive tract in 3 branches: One anterior to pharynx, 2 parallel posterior to pharynx.

PlatyhelminthesTrematodaOrder EchinostomatiformesFasciola

Habitat: Parasites in the bile passages of many kinds of mammals especially ruminants (sheep and cattle), humans occasionally infected.

Flute or Digenean. Adults feed on the lining of biliary ducts. Eggs are passed out of the liver with the bile and into the intestine to be voided with host feces. If eggs are shed into water they develop in larvae and enter the cell intermediate host. Additional development occurs within the snail and multiple parasites leave the snail and encyst on aquatic vegetation. Encysted vegetation is eaten by the final host (a mammal) and the life cycle is complete.


Habitat: Found in the intestine of most carnivores, dogs, cats, wolves, foxes, humans (parasitic)- the tapeworms.

Bidy consists of segments called proglottids. As proglottis enlarge and become older, they contain male and female reproductive organs which become functional (proglottids). Individuals 3-5 meters long have been reported.


Commensal in book gills of horseshoe crabs. Attach to the host using sticky secretions and a specialized posterior sucker. Females deposit their egg on the host's gills.


Habitat: Found on the lower shore coiled in writhing knots beneath boulders and on muddy sand. This species can often be found in rock pools entangled amongst laminaria holdfasts or in rock fissures. In deeper subtidal areas, it occurs on muddy, sandy, stony, or shelly substrata.

Unsegmented elongated ribbon worm. Common on east coast. Posses proboscis which is used in feeding.

AuricleChemical receptors; ears-like flaps; on Dugesia and Planaria; two pairs of ciliated paddle/tentacle-like structures unique to lobate ctenophores and assist in prey capture.
PharynxEverts out of mouth to suck up food.
Vans DeferensWhere sperm comes from.
Penis BulbConical mass inside genital atrium.
GonoporeWhere male and female reproductive systems come together and open to the outside.
Caudal SuckerBdelloura uses it to stick to gills of horseshoe crab.
EpidermisLayer of cells on external side of animal.
GastrodermisLayer of cells that lines the gut.
ParenchymaMesoderm; loose collection of cells; forms muscles.
MiracidiumFree swimming larva of Trematode Himasthla rhigedana; gutless and ciliated; eaten by intermediate host.
CercariaFree swimming larva; looks like sperm; get out of first intermediated host; encyst onto snail operculum or belly of crab (2nd intermediate hosts).
MetacercariaEncysted waiting stage on 2nd intermediate hosts.
Miller's LarvaRare planktonic larva of Class Turbellaria.
ProboscisFound on Phylum Nemertea; shot out by hydrostatic pressure of rhyncocoel; used prey capture; hollow.
RhynocoelWhere inverted proboscis lies; fluid filled tubular cavity; aka proboscis sheath.
StyletsAt ends of proboscis; spears prey; can secrete toxins.
Pilidium LarvaNemertean larva; feeding, swimming, long-lived (vs. Platyhelminthes).