The tapeworm has no mouth. It simply attaches itself to a host with suckers and hooks and then absorbs nutrients through its skin. The phylum Nematoda, possibly the most abundant phylum of animals on earth, contains all of the round worms. One characteristic of the round worms includes a psuedocoelom or false body cavity that houses all of the organs, acts as a circulatory system, and makes the worm rigid. This phylum is the first to have a digestive tract with two openings, an anus and a mouth.
Round worms are often quite parasitic.
One such parasitic round worm is the brain worm. This nematode is commonly found in deer, elk, moose, and the like. It is not harmful to the deer but will kill most of the other animals it normally inhabits. One experiment uses deer pellets to locate brain worms. Here on some pictures from the experiment.