CONTROL OF SENSE-ORGANS OVER PULSATION. 



13 



pigment spot but leaving the stalk of the sense-organ intact, the disk 

 is instantly paralyzed. Also, when the marginal sense-organ regen- 

 erates, regular pulsation is resumed as soon as the pigment spot and 

 a few small otoliths begin to appear. For example, figure 4 shows 

 the appearance of the normal sense-organ, and figure 5 the condition 

 of a regenerating sense-organ that has become capable of control- 

 ling the rhythm of the disk. Immediately after death the pigment of 

 the sense-organ dissolves out into the sea-water ; on the other hand 

 it appears remarkably stable in the living animal, and is not faded by 

 the most intense sunlight, nor changed by one month's confinement 

 in absolute darkness. 



Fig. 4. Fig. 5. 



Fig. 4. — Ejilarged views of a sense-organ of a mature Medusa of Cassiopea. A, aboral 



view ; B, side view ; C, oral view. 

 Fig. 5. — Enlarged oral view of a regenerating sense-organ, showing the beginning of the 



formation of pigment spot and otoliths. A wide strip of new tissue (dotted) separates 



the sense-organ from the old muscular layer of the sub-umbrella. 



If a sense-organ be cut out with the merest remnant of sub-umbrella 

 tissue left attached to it, examination under the microscope shows 

 that this tissue continues to pulsate rhythmically, and it is apparent 

 that the area of the sub-umbrella tissue attached to a sense-organ may 

 be reduced to a practical zero without any more marked effect than a 

 not very pronounced slowing of its rate of pulsation. On the other 

 hand, if we remove all but one of the sense-organs and then place the 

 disk in sea- water charged with CO2, keeping the sense-organ itself 

 out of the fluid, the disk becomes paralyzed and can not be enervated 

 into contraction by the sense-organ. In some of these experiments 

 the sense-organ was also paralyzed, although it had not been in the 

 CO2 solution. In others the sense-organ continued to send contrac- 

 tions out over the adjacent tissue, but these could not extend over the 

 parts of the sub-umbrella which were bathed by the CO2. 



