mon stock, from which are derived the Mollusca and Vertebrata. Supposing the Worms to be the origin of these four divisions of the animal kingdom, an interest is excited in them not exceeded by any other group. One must expect then to meet with great difficulties in making a satisfactory arrangement of these organisms, to meet with structures that foreshadow much more complete types, to find many similar forms as well as individuals so different as to make it nearly impossible to say whether they belong to the Worms at all. The classification as shown in Tree III. is principally that of Prof. Haeckel. It is true that this has met with objections, but it must be remembered that no tree could be constructed which would satisfy every anatomist. And this appears to be nearly in harmony with the views of the most eminent naturalists living. Bearing in mind that the nature of the subject will always make a classification with sharply-defined limits impossible, and as Haeckel says that this attempt, like every other of its kind, is provisional, to be corrected or confirmed by the specialists of the future, we will examine it a little more closely, being obliged, however, through the limits of this essay, to give only a general account. The Scolecida, or Soft Worms, supposed to have descended from Infusoria, divide into two groups,—the Flat Worms (Platyelminthes) and the Round Worms (Nematelminthes). The Round Worms include the horse-hair worms (Gordiaceae), so called from the superstition prevalent among country-people that the horse-hairs are changed into these worms. We find among them the Trichina spiralis, so famous in late years as the cause of the Trichiniasis, a disease resembling typhoid fever and acute rheumatism. The Trichina (Fig. 25) lives in its immature condition in a sac. These sac-like bodies are found in the pig, the meat of which, if not sufficiently cooked when eaten, will cause this very fatal disease, as the sac being destroyed by the juices of the intestine, the
