HETEROMORPHOSIS 157 



6. We have seen that in the uninjured animal it is the 

 foot which first bends downward on the wire screen arid 

 assumes a vertical position; while the head is the last to 

 assume this orientation. If an animal is cut across trans- 

 versely, and the two pieces are laid side by side upon the 

 wire screen immediately after the operation, the aboral frag- 

 ment carrying the uninjured foot begins to bend down 

 vertically sooner than the oral fragment which carries the 

 head. 



This difference in the irritability of the two portions of 

 the animal can be shown very prettily by making a trans- 

 verse incision at about the middle of the animal, so that 

 both pieces still hang together. If such an animal is laid 

 upon the wire screen, immediately after the operation, the 

 foot works itself through the rnesh in the net to the incision 

 and assumes a vertical position, while the oral piece extend- 

 ing from the incision to the heads usually remains lying hori- 

 zontally upon the wire screen. 



7. If the heads of Cerianthus, which have been cut off 

 close to the oral plate, and which no longer work their aboral 

 poles through the wire mesh when laid upon it, are laid upon 

 the sand for a time, they also at length assume a position in 

 which their long axis is in a vertical position. One receives 

 the impression at first that one is dealing with normal 

 animals buried deep in the sand. The method by which 

 they retain their vertical position is remarkable. Certain of 

 the cells of the ectoderm secrete a mucoid substance to which 

 kernels of sand become attached. But this secretion is 

 formed only on the base of the pieces which have been cut 

 off just below the oral plate. The kernels of sand which 

 adhere to the base have a greater specific gravity than the 

 animal itself, and this keeps the animal in an upright posi- 

 tion. 



8. All these experiments succeed equally well in the light 



