title for learn-about section of A SNAIL'S ODYSSEY

West-coast free-living marine flatworms are mainly represented by "turbellarians", with hundreds of species. However, many of these are microscopic in size or otherwise not readily accessible for study.  For these and other reasons, comparatively little research has been done on them. Members of the "Order" Polycladida are the most conspicuous marine representatives on our coast, with a number of species being found commonly in intertidal regions and on floating docks.  Most polyclads encountered are relatively small, but one species, Kaburakia excelsa, commonly exceeds 6cm in length.  Owing to the few research papers on west-coast polyclads, for convenience they are combined here with ribbon worms. The 2 groups resemble one another in features of body-wall construction, lack of segmentation, and carnivorous habits. Acoel turbellarians and parasitic representatives (trematodes and cestodes) are omitted from the ODYSSEY. Only the large-bodied polyclad group is considered.

Nemerteans, or ribbon worms, are conspicuous components of mussel beds and mud flats, and many species are symbiotic with decapod crustaceans and bivalves.  At least 50 intertidal and shallow subtidal species are described from west-coast shores, with about a dozen of these being commonly seen.  Although they have several morphological features in common with flatworms, nemerteans differ in usually having separate sexes, a gut complete with mouth and anus, and a blood circulatory system. Features of nemerteans include bright colours and patterns, extensible proboscis, and remarkable “stretchability”. 

NOTE the old "Class Turbellaria", with its handy progression of Orders from simpler (O. Acoela) to more complex (O. Polycladida), is now considered to be polyphyletic, with its representatives categorised into "clades"

  black dot
drawing of snail meeting flatworm from A SNAIL'S ODYSSEY
© 2010 Thomas Carefoot

To learn about west-coast FLATWORMS & RIBBON WORMS: select a topic from the flatworm/nemertean menu at the top of the page

OR: play the ANIMATION of the snail meeting the FLATWORM (on the Left)

OR, if you want to see other animations: follow the snail on its ODYSSEY by CLICKING on any X-marked invertebrate on the map (on the Right)

map for flatworm part of A SNAIL'S ODYSSEY
  black dot

Phylum Platyhelminthes (lit. “flat” “worm” G.)

Polycladida (lit. “many” “branches” G.), referring to the many-branched gut that is characteristic of the Order.  Although there are over 20 polyclad species recorded from Washington and British Columbia, the commonest ones seen intertidally are Notocomplana (Freemania) litoricola, several other species of Notocomplana, and occasionally Kaburakia excelsaEurylepta leoparda is found in association with solitary tunicates.

Phylum Nemertea (lit. “thread” G.), including ribbon worms (sometimes called bootlace worms, proboscis worms)

Palaeonemertea (lit. “ancient thread” G.), with unarmed (no stylet) proboscis and proboscis not differentiated into regions. The mouth is below or behind brain.  The commonest west-coast genus is Tubulanus.

Heteronemertea (lit. “different thread” G.), with unarmed (no stylet) proboscis and proboscis not differentiated into regions. The mouth is below or behind brain and is usually conspicuous.  Common west-coast genera include Cerebratulus, Lineus, and Micrura.

Hoplonemertea (lit. “armoured thread” G.), with proboscis armed with stylets and proboscis and proboscis differentiated, usually into 3 regions. The mouth is at the front and combined with the proboscis opening to make a single proboscis pore.  Common west-coast genera include Paranemertes, Amphiporus, Carcinonemertes, and Emplectonema

  black dot