I/O C. M. CHILD. 



The heads of whole worms in anaesthetics gradually assume 

 the shape characteristic of heads regenerated under the same con- 

 ditions. The auricles decrease in length until scarcely visible, 

 the head becomes smaller in proportion to the body and its out- 

 lines become more rounded. 



In many cases, though by no means always, the large indi- 

 viduals begin after a few days in the anaesthetics to degenerate in 

 the region representing the posterior end of the first zooid, i. e., 

 the anterior half of the postpharyngeal region, and this degenera- 

 tion results in the complete separation of a posterior portion 

 corresponding to the second zooid and an anterior portion con- 

 sisting of the first zooid minus more or less of its posterior end. 

 In some cases the disintegration proceeds gradually in each piere 

 after separation until the whole is disintegrated, but very com- 

 monly it ceases after the separation of the two parts, the surfaces 

 heal and the posterior piece develops a new head as it would do 

 after normal fission, while the anterior piece produces a small 

 amount of new tissue at its posterior end. This process of disin- 

 tegration is not due to an infection or to any other accidental con- 

 dition in the medium, as is clearly shown by two facts : first, it 

 increases in frequency with decreasing temperature. At low tem- 

 perature practically every large individual separates into two 

 parts while in room temperatures separation often occurs in less 

 than half. And second, such disintegration and separation almost 

 never occurs in individuals below a certain size, either in high or 

 low temperatures. In short we find that the posterior region of 

 the first zooid in large individuals is subject to degeneration and 

 disintegration in anaesthetics, and more frequently in low than in 

 high temperatures, while in small individuals degeneration rarely 

 or never occurs in this region. 



I can account for these facts only as follows : In the large indi- 

 vidual the first zooid is approaching the limit of size and its pos- 

 terior end being most distant from the centers of correlation, the 

 cephalic ganglia, is approaching a condition of what we may call 

 physiological isolation. When we place such individuals in the 

 anaesthetics we decrease the effective distance of transmission of 

 the nervous processes and the posterior region of the first zooid 

 becomes still further isolated. This region is incapable of form- 



