FORM-REGULATION IN CERIANTHUS. 267 



duction comprises two distinct stages. During the first stage the 

 collapsed tentacles gradually become shorter but do not change 

 in appearance, except to become more and more deeply colored 

 as the pigment-particles are more and more closely approximated 

 with reduction in surface. This reduction continues during a 

 month or more according to various conditions, c. g., the com- 

 pleteness of collapse, the temperature, etc. 



If, however, the condition of collapse be continued during a still 

 longer period the tips of the tentacles began to atrophy, becoming 

 darker in color and shrivelled or withered in appearance. This 

 process of atrophy gradually extends proximally along the ten- 

 tacle if the internal pressure is not reestablished, until in some 

 cases mere stumps of tentacles remain. The Figs. 3-5 show 



FIG. 3. FIG. 4. FIG. 5. 



schematically the process of atrophy, one side of the disc with 

 tentacles being shown in section in each figure. Fig. 3 repre- 

 sents a stage in which the marginal tentacles have been reduced 

 by about one half their length and the labial tentacles relatively 

 less. Figs. 4 and 5 represent later stages. In Fig. 5 the long 

 shrivelled distal portion seen in Fig. 4 has been almost com- 

 pletely resorbed. The reduction in size of the disc in con- 

 sequence of reduced pressure is also indicated in the figures. The 

 difference in appearance between the healthy and shrivelled por- 

 tions is always marked and visible at a glance. The tips are 

 always much darker in color than the normal tentacle and are 

 irregular in contour, so that the use of the term " shrivelled " is 

 amply justified by their appearance. The proximal healthy 

 portions are about equal in length in all the tentacles of each set, 

 /. e., the atrophy of the tips proceeds with equal rapidity in all ten- 

 tacles of each set. The tips apparently always remain closed. 



