5O C. M. CHILD. 



the incision was proximal to the level of the ostia then all the 

 chambers distal to the incision are in communication with each 

 other and with the enteric cavity in general. In the first case 

 distension of other portions of. the body with water has no effect 

 upon this region, but in the second, distension of other parts is 

 accompanied by distension of this region. 



In Harenactis, as in Cerianthus, a considerable degree of disten- 

 sion exists when the animal's are undisturbed, consequently body- 

 wall, tentacles and mesenteries are subjected to a certain degree 

 of mechanical tension as a characteristic feature of normal life. 

 The following experiments show very clearly the importance of 

 this distension as a factor in determining the persistence or atrophy 

 of parts. * 



Fig. 3 is a diagrammatic longitudinal section of an animal ten 

 days after a lateral incision just distal to the level of the ostia (a, 

 Fig. i). Tentacles have developed proximal and distal to the 

 opening as usual, but the whole region distal to the opening is 



FIG. 3. FIG. 4. 



much less distended than other parts of the body. The old ten- 

 tacles at the distal end of this region are undergoing atrophy, as 

 is indicated by their length as compared with the tentacles of the 

 other side of the body and by the degenerating region at the tip : 

 the heteromorphic tentacles at the proximal end of this region 

 are small and short ; on the other hand, the tentacles proximal 

 to the incision are nearly as long as the old tentacles on the other 

 side of the body. 



