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PRECAMBRIAN PERIOD

630-600 million years ago

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worm digestive tract

Higher worms moved their mouth to the head. Their descendants evolved a complete digestive tract which eliminated wasts through an anus.

While some flatworms have anal canals, most have only one opening of their digestive tracts (Beklemishev 2). Advanced flatworms possess a diverse set of digestive glands although their products aren't well known (Rieger, from Harrison, 1991). Nemertine worms show a number of advances over flatworms, to which they are thought to be related. They possess a complete digestive system, with a mouth on one end and an anus on the other. In a few species, the mouth is not at the anterior end of the animal and is located more posteriorly. Food is moved primarily through ciliary action and is digested both extracellulary and intracellulary. In some, the larval forms lack an anus, as is the condition in flatworms (Hickman, p. 227).