FORM REGULATION IN CERIANTHUS. 26/ 



said here that Loeb failed to understand the real conditions, 

 otherwise he would scarcely have attempted to apply the osmotic 

 hypothesis to this case. 



The evidence bearing upon the relation between regeneration 

 and internal water-pressure is varied in character. In the re- 

 maining portion of the present paper and in following papers the 

 various lines of evidence will be discussed. First, however, it is 

 necessary to call attention to certain features of the water-pres- 

 sure and circulation in the enteron of normal animals. Except 

 where statement to the contrary is made all observations and 

 experiments were made upon C. solitarius. 



WATER-PRESSURE AND CIRCULATION IN THE ENTERON. 



Under ordinary conditions both the body-wall of Ceriantlnis 

 and the tentacles are subjected to a considerable degree of ten- 

 sion in consequence of internal water-pressure, the enteric cavity 

 being distended with water. When contraction occurs this water 

 issues through the aboral pore, at least in large part, though some 

 passes out through the stomodaeum. After loss of water from 

 the enteron the body and tentacles are shorter and smaller, and, 

 if the loss was great, are more or less completely collapsed. This 

 condition continues until the enteron again begins to fill with 

 water. As it fills, the body gradually resumes the original form 

 exactly as does a rubber balloon when inflated with air. This 

 distension of body and tentacles by water under pressure is an 

 indispensable condition of the characteristic form of body and 

 tentacles. The animal is incapable of extending to its full length 

 or attaining its full diameter in any other way than through the 

 medium of the internal water-pressure. Extension of the tenta- 

 cles and the turgid condition often designated as erection is just 

 as completely the result of the enteric water-pressure. Turgor 

 of individual cells has absolutely nothing to do with the condi- 

 tion, as is evident to all who have examined with any care Ccri- 

 anthns or other actinians. Loeb's belief ('91) that osmosis plays 

 a part here is wholly without foundation, except so far as water 

 may diffuse through the body-wall into the enteron. 



We may regard CcriantJins as a sac filled with water under 

 pressure. Under certain conditions the walls of the sac may de- 



