2 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
sub-umbrella floor of the ocular lappets ; for even in very large medusæ 
the ocúlar lappets exhibit only a slight notch, as is seen in oc, oc, Fig- 
ure 6, and in Figure 26, Plate X., and only in a very few of the largest 
and most mature medusæ do we find the ocular lappets divided so that 
the tertiary tentacles arise from the cleft. This latter condition is shown 
in Figure 19, Plate IX. It is interesting to notice that these tertiary 
tentacles do not make their appearance until the medusa is almost 
mature, when the bell has attained a diameter of 130 mm., and even 
then many of them fail to develop into anything more than mere fila- 
ments of very short length, such as are shown in Figure 4, Plate IV., 
and Figure 27, Plate IX. Indeed they never grow to a greater length 
than is seen in Figure 6, which represents their condition in a medusa 
measuring 190 mm. across the disk. These rudimentary tentacles stand 
in striking contrast with the primary and secondary ones, which, when 
fully extended, attain a length of from three to four times the diameter 
of the disk. 
In common with all other Pelagidæ, there are eight marginal sense 
organs in Dactylometra ; four of these occupy the primary, and four the 
secondary radii. They are set into little niches in the edge of the bell, 
and project downwards from the oral surface. Figure 7, Plate VIL, 
represents the sense organ as seen from the oral side, and Figure 8 is a 
somewhat diagrammatic longitudinal section, showing the so called 
“olfactory pit” (opt), which projects downwards from the upper surface 
of the bell just above the region of the sense organ. As was long ago 
pointed out by L. Agassiz, these sense organs are morphologically noth- 
ing more than little hollow tentacles, the entoderm of which contains 
a mass of otolythic concretions (con, Figs. 7, 8, and 9). 
The mouth opening (M, Figs. 6, 31, Plates VI., IX.) is cruciform, 
and occupies the centre of the oral surface of the disk. It is surrounded 
by four oral fringes, or palps, which occupy the secondary radii, and 
when fully expanded attain a length of about three or four times the 
diameter of the disk itself. Fewkes! is mistaken in stating that the 
oral appendages are “ of two kinds, four of which are quite long, floating 
gracefully along after the medusa as it swims in the water, The re- 
maining oral appendages being shorter, more ruffled, and confined to the 
immediate vicinity of the mouth.” He was evidently deceived by 
observing a peculiar state of contraction of the oral fringes, for their 
shape is constantly changing. Sometimes one sees them as beautifully 
1 Fewkes, J. W., 1881; Studies of the Jelly Fishes of Narragansett Bay, p. 178 
Bull. Mus. Comp. Zoöl., Vol. VIII. pp. 142-182, 10 Plates. 
