REGENERATION OF LOST PARTS.—The process of budding and fission of the worm into two parts is merely an extension of that resulting in the formation of new segments when the worm is injured. In most of the Nereidiform Polychaetes the number of segments forming the body continues to increase throughout life by the formation of new segments between the anal segment and the one in front of it; that is to say, there is normally a process of budding taking place at this point. Now in many of the longer worms it may be noticed that the segments of the hinder end suddenly become smaller than the rest; these are segments newly formed to replace those lost by the worm. But this "regeneration," though the same in principle as ordinary growth {283}at the penultimate segment, is due to activity in a segment (any segment) further forwards; in other words, in the less modified worms every segment has the power of forming new tissues, just as each of the joints of a crab's leg has the power of forming the remaining joints when injured. It is not therefore surprising that a "zone of budding" arises in an uninjured worm at certain seasons, viz. that of reproduction; it is a property that each worm possesses, though generally it remains latent till injury provides the stimulus. Moreover, not only can new segments arise at the hinder end, but a new head can be formed at the anterior end, as has been observed in worms belonging to many families—in the less modified Syllidae,[338] in others of the Nereidiformia, and even in Sabellids, where the greatly specialised gill filaments can be reproduced. Thus Sir J. Dalyell[339] noted in _Dasychone_ that the crown of branchiae was regenerated in about a month in springtime, while in winter the process occupied 116 days. He cut a _Dasychone_ into three pieces; the hindermost produced a head, the anterior piece developed an anus, and the middle portion formed both a head and tail! These regenerated heads are of course at first smaller than the rest of the body, but soon grow to a normal size. Naturally this extensive power of regeneration is of extreme value to the Polychaetes, for if a fish or other enemy bites the head off a worm, a new one can form; and it is not difficult to see in this the origin of the reproduction by fission as a normal process. {284}CHAPTER XI NATURAL HISTORY OF POLYCHAETES—GENERAL HABITS—CHARACTER OF TUBE AND ITS FORMATION—COLOURING—PROTECTIVE AND MIMETIC DEVICES—PHOSPHORESCENCE—FOOD —USES—ASSOCIATED WORMS—WORMS AS HOSTS—DISTRIBUTION—FOSSIL REMAINS.