I5O C. M. CHILD. 



which is employed in closed pieces in producing increase in 

 length of the tentacle, may, in the absence of the usual power- 

 ful stimuli to longitudinal growth, be used in regions where 

 stimuli are less powerful and so in consequence of the general 

 pressure in all directions bring about an atypical increase in the 

 transverse diameter of the tentacle. According to this view the 

 stimulus to longitudinal growth in closed pieces is so much 

 stronger than the others that growth occurs almost wholly in 

 this direction ; this stimulus being reduced others become more 

 effective. The reactions of various forms, both growth-reactions 

 and motor- reactions, in response to stimuli might be cited in 

 support of this view, for it is well known that in many cases the 

 predominant stimulus determines the reaction. 



Attention has already been called to the fact that in the open 

 pieces with delayed tentacle-regeneration the tentacles usually 

 acquire an arrangement in two or three rows much earlier than 

 in the closed pieces with typical regeneration. I believe that this 

 difference is due to the smaller circumference of the disc and the 

 rounded, more or less sac-like form of the tentacles in open pieces. 

 It is clear that the smaller the disc the fewer the tentacles which 

 can be borne in a single row upon its margin ; and when the 

 tentacles themselves are less slender on pieces with the smaller 

 discs than on the typical pieces, it follows that the crowding of 

 some of them out of the single row must occur all the earlier. 

 Figs. 2 and 4, as compared with Figs. 3 and 5, are good 

 examples of this difference. In the closed, typical pieces, where 

 the disc begins to expand as soon as regeneration begins, the 

 tentacles are found mostly in a single row until a comparatively 

 late stage of regeneration, when they are gradually forced into 

 two or three rows. In the open pieces, on the other hand, the 

 blunt, sac-like tentacles often form two more or less complete 

 rows a short time after their first appearance. This difference 

 between open and closed pieces, which can scarcely be explained 

 on any other basis, affords very strong evidence in favor of the 

 view that the arrangement of tentacles in the normal animal in 

 about three rows is the result of mutual pressure. It is possible 

 that slight differences in the circulatory currents, themselves due 

 to differences in lengths of mesenteries or other structural or 



