If we examine the method of regeneration from the posterior end of a piece of an earthworm, we find that when several or many posterior segments have been removed a new part comes back, composed at first of a very few segments. The terminal segment contains the new posterior opening of the digestive tract. New segments are now formed just in front of the terminal segment, the youngest being the one next to the end-segment. The process continues until the full complement of segments is made up (Fig. 3, _C_, _D_, _E_). Comparing these results with those described above for the anterior end, we find, in both cases, that only a few segments are at first formed, but in the posterior regeneration new segments are intercalated near the posterior end. This process of intercalation is the characteristic way in which many annelids add new segments to the posterior end, as they grow larger and longer. Amongst the flatworms the fresh-water planarians show remarkable powers of regeneration. If the anterior end is cut off at any level, a new head is produced (Fig. 4, _C_). The new worm is at first too short, _i.e._ the new head is too near the pharynx, but changes take place in the region behind the new head that lead to the development of new material in this part. The new head is, in consequence, carried farther and farther forward until the typical relations of the parts have been formed, when the growth in the region behind the head comes to an end (Fig. 4, _C¹_). Similar changes take place when the posterior end is cut off, as shown in Fig. 4, _B, B¹_. The new part contains the new pharynx that is proportionately too near the head, but the pharynx is carried farther backwards by the formation of new material in front of it, until it has reached its typical distance from the head. In these planarians the results are somewhat complicated, owing to the old part changing its form, especially if the piece is not fed; but the main facts are given above, and a more complete account of the changes that occur will be given in another place. [Illustration: FIG. 4.--_A-E. Planaria maculata._ _A._ Normal worm. _B, B¹._ Regeneration of anterior half. _C, C¹._ Regeneration of posterior half. _D._ Cross-piece of worm. _D¹, D², D³, D⁴._ Regeneration of same. _E._ Old head. _E¹, E², E³._ Regeneration of same. _F._ _P. lugubris._ Old head cut off just behind eyes. _F¹._ Regeneration of new head on posterior end of same.] _LATERAL REGENERATION_