Jessesmom -- Your large flatworms?

Hyman (1943) described Dolichoplana striata Moseley. "Length up to 120mm. Form very elongated and flattened. Ground color yellowish brown with 6 longitudinal dark stripes, paired median, lateral, and marginal. Median stripes are very narrow and may be absent in young specimens. Lateral stripes are very conspicuous, black, sharply defined, beginning at level of eyes and continuing to posterior tip. Marginal stripes are less dark, diffuse, ill defined, fading away posteriorly. Creeping sole white, bordered on each side by diffuse pigmentation."

Bipalium kewense is recognized by its pale color, dorsal dark median line, dark patch in neck region, and expanded lunate head. This species may attain lengths up to 30 cm. (Chandler 1974).

BTW, have you taken any of those to your vet's lab for examination? That might be an option.

Came across this info too:

http://www.britannica.com/eb/topic-489506/rag-worm

rag worm, or bristle worm, or clam worm, or mussel  worm, or Nereis, or sandworm (annelid)

any of a group of mostly marine or shore worms of the class Polychaeta (phylum Annelida). A few species live in fresh water. Other common names include mussel worm, pileworm, and sandworm. Rag worms vary in length from 2.5 to 90 cm (1 inch to 3 feet); they are commonly brown, bright red, or bright green. Rag worms are perhaps the most highly developed of the annelids.

http://www.peteducation.com/article.cfm?cls=2&cat=1621&articleid=749

The giant kidney worm, Dioctophyma renale, most commonly lives in mink. Rarely dogs, cats, fox, and other carnivores may be infected. There are rare reports of D. renale infecting humans. D. renale is found throughout the United States.

A D. renale infection is most commonly found on a post mortem examination. Occasionally, the worm may be found during abdominal surgery for some other condition. Imagine the look on the veterinarian’s face upon finding a 3-foot long worm during a 'routine' spay!

D. renale is quite selective. It is almost always found in the right kidney.
 

(Although these are typically found in the kidney, who’s to say that they can’t be in the gut too.)