=Classification.=--There is great lack of agreement among zoologists in the matter of the classification of the worms. Not only are the various groups which by some are called classes held by others to be distinct branches, co-ordinate in rank with the Echinodermata, Cœlenterata, etc., but the limits of these groups are also constantly called in question. It will require a great deal better knowledge of the structure and life-history of these diverse animals before the matter of their classification is satisfactorily settled. We shall consider briefly four of the various groups (which we may consider as classes) which include worms either specially familiar to us or of special interest or importance. One or two examples of each group (the groups being selected primarily because of the examples) will be described in some detail. By this means we may get an idea of the extremely diverse character of the animals which are included in the heterogeneous branch Vermes. =Earthworms and leeches (Oligochætæ).=--The various species of earthworms, an example of which has been studied are found in all parts of the world; they occur in Siberia and south to the Kerguelen Islands. They are absent from desert or arid regions, and some can live indifferently either in soil or in water. Some near allies of the earthworms are aquatic, living in fresh or brackish water, some in salt water near the shore. In size earthworms vary from 1 mm. (1/25 in.) to 2 metres (2-1/6 yds.) in length. All show the distinct segmentation of the body noticeable in the common earthworm already studied. The leeches, some of which are familiar animals, are closely related to the earthworms, although at first glance the similarity in structure is not very noticeable. TECHNICAL NOTE.--Some common water-leeches, alive or preserved in alcohol, should be examined by the class. The animals are not unfamiliar to boys who "go in swimming" in the small streams of the country. The body of a leech should be examined carefully, and drawings of it showing the external structural characters should be made.