AGASSIZ AND MAYER: DACTYLOMETRA. 5 
an aquarium for a few days they lose their bright color and fade into a 
dull yellow. Indeed, if the medusa be kept in an aquarium without 
food for about six weeks, its disk shrinks to about one eighth of its 
former diameter, and all color fades away until it becomes transparent. 
The aboral surface of the disk is thickly sprinkled over with light ochre- 
yellow spots, caused by protuberances bearing masses of nettling cells ; 
and in addition to these there are sixteen radially arranged regions of 
reddish brown spots, lying just above the radial septe of the stomach 
(see Figs. 2, 3, and 5). These reddish regions usually extend for about 
half way from the periphery of the disk towards the centre, and a closer 
examination shows that they are caused by highly refractive rosin 
colored pigment granules which are contained in the protoplasm of the 
epithelial cells of the disk (see Fig. 20, Plate IX.). In this respect 
indeed, according to McKendrick,! they are similar to the red pigment 
spots of Chrysaora. The male genital organs are usually decidedly pink 
in color, while the ovaries are often yellowish, or ashy gray. The bands 
of muscles in the radial septæ are of a glistening white. The ectoderm 
of the tentacles is ochre-yellow, and the entoderm is often pink. The 
oral fringes are often yellowish, with a delicate shade of opalescent blue 
in certain lights. In other individuals, however, they are of a delicate 
shade of pink, and they are always sprinkled over with red colored 
pigment spots similar to those of the aboral surface of the disk. The 
marginal sense organs are usually white in color. 
The habits of this medusa have already been described by A. Agassiz,” 
who says: “It is somewhat strange that almost all of the Medusæ which 
have been observed were found in the brightest sunshine only, or in very 
dark nights. Early in the morning, and until about ten o'clock, even 
on clear days, meduse do not make their appearance, while from eleven 
until one or two o’clock they can be caught in abundance. After this 
time they disappear gradually, and late in the afternoon it is rare to see 
a single jelly-fish. Between nine and ten o’clock they come to the sur- 
face again; and that hour, in fact, is one of the most favorable for col- 
lecting, in spite of the darkness.” When the medusæ are confined in 
the stale water of an aquarium they often assume the position shown in 
Plate V., where the disk is flattened out to an extreme degree, the palps 
are spread out over the floor of the aquarium, and the tentacles droop 
listlessly downward. In this position the medusa often remains 
1 McKendrick, J. G.: Coloring Matter of Medusa. Journ. of Anat. and Physiol., 
1881, Vol. XV. pp. 261-264. 
2 North Am. Acal., 1865, p. 49. 
