Among the lowest forms of worms are the Planarian worms, already alluded to as examples of the third grade of animal existence. These belong to the class Turbellaria, which is represented by plenty of both fresh water and marine forms in our own country and on its coast. The Turbellaria are divided into groups called Acoela, Dendrocoela, and Rhabdocoela. These names allude to the intestine, which in the first group is wanting, in the second branched like a tree, and in the third straight. The Cestoda or tape-worms, which absorb nourishment through the skin, and therefore need no alimentary canal, and possess none; and the Trematodes, represented by the Liver-fluke, which infests sheep, together make up the group of flat-worms (Platyhelminthes), of which mention has already been made (p. 44). In all of them the body is more or less flat, and the digestive cavity, like that of Coelenterates, has but one opening, the mouth. The life-history of parasitic worms is described in a well-known volume by Leuckart, which forms the basis of our knowledge on the subject. Since its publication, discoveries regarding parasites have been constantly added by other observers. The history of the Liver-fluke is a most complicated example of alternation of generations. The adult form infests the sheep's liver. There it produces eggs, which afterwards find their way into water. Here they die unless they find their way into a certain water-snail, which many of them do. Within this snail--_Linnæa truncatula_--the egg develops into a sac-like body, called a sporocyst. This produces within itself numbers of a small creature which is called the Redia form. These in turn produce a tailed form, called a Cercaria, which gets out of the snail, swims in water, and finally settles down on some plant. Here it is eaten by an unfortunate sheep, within which it develops into the adult fluke. The other great divisions of the Vermes are as follows: The Nematodes or thread-worms, a group of parasites which includes the dreaded _Trichina_; the Nemertines, a group mostly carnivorous, possessing a curious proboscis, and often an armed skin; the Leeches or Hirudinea, and finally, the Chætopods (Bristle-footed Worms), the highest group of all, containing the forms often spoken of as Annelides--_i.e._ Ring-shaped Worms.