40 



C. M. CHILD. 



entirely from the oesophageal region, in which the oesophagus and 

 body-wall unite at both ends and there is no communication be- 

 tween the enteron and the exterior. The distension which occurs 

 during the first few days after closure disappears later and com- 

 plete collapse takes place. Under these conditions atrophy of the 

 tentacles is very rapid : in some cases complete disappearance 

 of fully developed normal tentacles occurred within thirty days. 

 Tentacle-atrophy is always followed in these pieces by atrophy 

 of the body- wall and finally fragmentation and death of the 

 pieces. 



Partial tentacle-atrophy can be induced in this species merely 

 by keeping the animals in dishes without sand in which they can 

 burrow. Without the support of the wall of the burrow the 

 body-wall is unable to sustain the normal degree of internal 

 pressure and the tentacles undergo partial atrophy in conse- 

 quence of the altered conditions. Apparently the internal pres- 

 sure is regulated to a greater or less extent by the strain upon 

 the body-wall, for it is certain that in animals outside the bur- 

 rows the internal pressure is not nearly as great as when the 

 body-wall is supported by the wall of the burrow. So far as I can 

 determine this difference is not due merely to escape of the water 

 through the aboral pore when a certain degree of pressure is 

 reached, for the pressure is always much below the point when 

 opening of the aboral pore occurs : there seems rather to be a 

 regulation of the entrance of water. Occasionally for some 

 reason regulation does not occur or is insufficient and rupture 

 of the body-wall takes place, usually in the aboral third, where 

 the body-wall is thinner than in other regions. 



After animals have been kept for five or six weeks under these 

 conditions the marginal tentacles are usually 10-15 mm. and the 

 labials about 5 mm. in length. 



A marked difference in the length of tentacles produced in 

 regulation likewise appears according as the animals are allowed 

 to burrow or are kept without sand. Tentacles developing on 

 pieces which live in burrows may attain the length of the origi- 

 nal tentacles while those on pieces without sand reach only half 

 this length or less. 



The body-wall of C. solitariiis is much thicker and more resist- 



