No. 569] REGENERATION 283 



arrangement of some of the more important structures 

 studied in this experiment. The dissection was made by 

 making a longitudinal cut in the body wall a little to the 

 left of the mid-ventral line, and then pulling the flaps 

 apart and pinning the animal down on its dorsal surfa»v. 

 The Polian vesicle is shown attached to the ring canal in 

 the position where it is usually found when only one is 

 present, that is in the left dorsal interradial space. Tt 

 will be noticed that the retractor muscles are simply 

 branches of the longitudinal muscles, and hence are radial 

 in position. At the time of evisceration the body wall 

 breaks a short distance posterior to the tentacles, the re- 

 tractor muscles separate at the point where they join the 

 longitudinal muscles and the intestine breaks off just in 

 front of the cloaca. 



A better understanding of the radial type of structure 

 will be gained by a reference to Fig. 2. This figure is a 

 diagram to show the relation of the radial to the bilateral 

 symmetry. The dorsal side of the animal is represented 

 toward the top of the page, the esophagus appears in 

 cross-section, cut just anterior to the stomach, and there- 

 fore one is looking forward to the other organs shown. 

 The retractor muscles, showing the position of the radii, 

 are much contracted and thickened, a condition in which 

 they are usually found after evisceration. The stone 

 canal ending in the small madreporite is located in the 

 mid-dorsal interradial space. Passing around in a clock- 

 wise direction, the other interradial spaces are designated 

 as right dorsal, right ventral, left ventral and left dorsal. 

 Polian vesicles may be found in any of the interradii ex- 

 cept the mid-dorsal space which always bears the stone 

 canal. Although only one Polian vesicle is represented in 

 this figure, the mid-ventral retractor muscle is shown 

 double, a split condition which is characteristic when two 

 or more Polian vesicles are present. This description 

 will be sufficient to show the general relation between the 

 radial and the bilateral symmetry. 



