A Case of Regeneration in Polycluete Worms. 



333 



In cases of regeneration my experience has been that new segments are 

 much more freely produced at the posterior than at the anterior extremity of 

 the body, and the problem which occurred to me was, how, and in what length 

 of time, the thoracic segments (about eight in number), with the inverted 

 arrangement of chaetal appendages, which is needful to the life-work of the 

 worm, would be reproduced. The answer came in the nature of a surprise. 

 Beside the cephalic plume-bearing segment, one new setigerous thoracic 

 segment only was formed, but the chaetal plan of the succeeding five or nine 

 abdominal segments was changed ; the dorsal uncini in these segments first 

 disappearing gave place to setae, and later the ventral setae were replaced by 

 uncini ; the new setae and uncini, moreover, were changed to the forms 

 characteristic of this part of the body. In other words, so far as the chaetal 

 plan is concerned, a new thorax had been constructed from the abdominal 

 segments. How far the internal structure has been affected by the change 

 remains to be ascertained. 



The observations extended over a period of five weeks, and were made 

 upon two portions of apparently one and the same worm. One portion 

 (comprising the two parts marked a and c on the figure), about f inch long 

 and consisting of 69 abdominal segments, being minus head and thorax, as 

 well as the anal, and numerous preanal segments ; the other part, b, £ inch 

 long (probably the hinder portion of c), consisting of 36 preanal and the anal 

 segment. 



In order to expose a large surface of water to the air and bring the animals 

 as near thereto as possible, the experiments were carried on in watch-glasses, 

 fresh sea-water being supplied twice daily. The first fragment, ac, in course of a 

 day or two, attached itself to the watch-glass by means of a narrow cord or loop 

 which it secreted, and which served, by a constant twisting movement of the 

 animal, to sever the portion, c, § inch long, and consisting of 18 of the posterior 

 segments. The number of parts available for observation was thus increased 

 to three. Each was placed in a separate watch-glass, to which it attached 

 itself slightly by a secretion from the ventral surface, and each part succeeded 

 in changing the chaetal arrangement of certain abdominal segments into that 

 of thoracic ones, as follows — in a, nine segments became thoracic ; in c, five ; 

 and in b, nine, The normal number of thoracic segments in the few adult 

 specimens which have passed through my hands is eight (in one case it is 

 nine), but the number appears to vary considerably in any given species of 

 Sabellid. 



The number of abdominal segments transformed is possibly, to some extent, 

 regulated by the total number of segments contained in the fragment under- 

 going repair, but it may also be dependent upon the number of thoracic 



