young Trichina is set free, and deposits eggs, the embryos from which bore through the viscera of the unfortunate one. The Trichina belongs to the family of thread-worms or Nematoda. Associated with the horse-hair and threadworms is the Echinorhynchus, or Bristly-snout (Fig. 27), so called from the proboscis or snout (Fig 27, a), armed with recurved hooks, being the most striking feature in its organization. The three families of Round Worms live within other animals,—the Trichina in the pig and man, the Echinorhynchus in the flounder, the Gordius, or horse-hair worm, in insects. They are sufficiently alike in their general organization to be the descendant of a common ancestor; their modifications are due to their different modes of life. Turning now to the Flat Worms, we see, according to Tree III., that the Turbellaria are probably the oldest of this group. They are arranged in two families, Dendrocoela and Rhabdocoela, according as the intestine is branched, as in the Planaria (Fig. 28), or straight, as in the Prorhynchus (Fig. 29) or Vortex. The Planaria are found principally in fresh water, but also in the sea, adhering to stones or stems of plants. In the haired covering of their bodies, in their internal organization and embryo forms, the Turbellaria are closely allied to the Infusoria, from which, most likely, they have been derived. From the Turbellaria have retrograded the Trematoda, of which the liver fluke (Distoma) (F'ig. 30) is a common example. By retrograding, I mean that the Flukes have lost organs, from want of use, which they would have retained had they not lived as parasites in the viscera of other animals. The Flukes are found in almost every kind of animal. Imagine a long chain of Flukes living as one individual! and we have the Tape-worm, or Taenia (Fig. 31), as a representative of the Cestoda. Either the Tape-worm has split into Flukes, or the Flukes have colonized and made the Tape-worm, or possibly they are both aborted Turbellarians.
