HETEROMORPHOSIS 157 
6. We have seen that in the uninjured animal it is the 
foot which first bends downward on the wire screen and 
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 mesh 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 isin 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 
Digitized by Microsoft® 
