42 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
The method of ascertaining whether this sense 
is lodged in the marginal bodies was, of course, 
extremely simple. Choosing a dozen of the most 
vigorous specimens, I removed all the marginal 
bodies from nine, and placed these, together with 
the three unmutilated ones, in another bell-jar. 
After afew minutes the mutilated animals recovered 
from their nervous shock, and began to swim about 
with tolerable vigour. I now darkened the room, 
and threw the concentrated beam of light into the 
water as before. The difference in the behaviour 
of the mutilated and of the unmutilated specimens 
was very marked. The three individuals which 
still had their marginal bodies sought the light as 
before, while the nine without their marginal bodies 
swam hither and thither, without paying it any 
regard. 
A further question, however, still remained to be 
determined. The pigment spot of the marginal 
body in Meduse is, as L. Agassiz observed, placed in 
front of the presumably nervous tissue, and for 
this reason he naturally enough suggested that if 
the marginal body has a visual function to perform, 
the probability is that the rays by which the organ 
is affected are the heat-rays lying beyond the range 
of the visible spectrum. Accordingly I brought a 
heated iron, just ceasing to be red, close against the 
large bell-jar which contained the numerous speci- 
mens of Sarsia; but not one of the latter approached 
the heated metal. 
From these observations, therefore, I conclude 
that in Sarsia the faculty of appreciating luminous 
