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. Enlarged 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 CO 2 , keeping the sense-organ itself 

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

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

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

 C0 2 solution. In others the sense-organ continued to send contrac- 

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

 parts of the sub-umbrella which were bathed by the CO 2 . 



