This being the case we can hardly give a satisfactory summary of the characteristics of the group, and therefore it must be understood that in our attempt to do so we unavoidably exclude some forms that belong to it according to our present system of classification. This being remembered, we will define worms as soft-bodied and elongated animals, exhibiting a bilateral symmetry (that is, having appendages and organs arranged symmetrically on each side of a plane extending from the dorsal to the ventral surface through the centre of the body), and with the body usually divided into a succession of segments, each of which resembles the one preceding and following it. Though many of the worms are generally looked upon as uninteresting creatures, of such an unattractive appearance and with such depraved habits that they are beneath respect, yet a study of the sub-kingdom will prove that not only does it include a number of wonderful forms with the most marvellous life histories, but that some of them are very beautiful objects; and this last remark refers more particularly to many of the marine worms, which come directly within the scope of our work. Before passing on to the special study and classification of the marine species, however, we must say a few words concerning the worms in general, reminding the reader that all our statements regarding the anatomy of the creatures may be readily verified by simple dissections of one or two typical species, such as the common earthworm, the fisherman's lugworm, the sea mouse, or the common horse-leech of our fresh-water ponds. With this object in view, the animal may be killed by immersion in spirit, then pinned out in the dissecting tray under water, and the body-wall opened by means of a pair of sharp-pointed scissors. The digestive tube of a worm runs completely through the length of the body, and though there is no distinct head, there is always a mouth, and this is often provided with horny jaws, and sometimes also with horny teeth, with which the animal is enabled to inflict wounds on its prey.