32 TROPICAL PACIFIC ECHINI. 
from different sides. They are drawn from a dried specimen, the sacs having 
been treated with alcohol and glycerine. The cystacanths of P. atrata differ 
greatly from those of P. pedifera. The sacs are more closely compressed, 
forming a club at the top of the rod (Pl. 2, figs. 1-3). The bladders contained ' 
in the sacs are much thicker than those of P. pedifera. The sacs are also 
irregularly dotted with black spots. The cystacanths of P. atrata are found 
on the abactinal extremity of the ambulacrum, close to the ocular plates, and 
are absent from the actinal side. Pl. 1, figs. 7, 2 are drawn from an alcoholic 
specimen. 
In Colob. Stimpsoni the cystacanths are comparatively larger (Pl. 2, 
figs. 4-7), but coming from a dried specimen they have lost their globular 
shape and the existence of pores could not be traced. The sacs seem tougher 
than those of P. pedifera. In one of the figures (Pl. 2, fig. 5) a few spicules 
could be detected. 
The cystacanths of Colob. Mertensii (Pl. 2, figs. 8-12) are elongate, re- 
sembling more those of Colob. Stimpsoni. They showed no pores. The spines 
of the cystacanths (figs. 11, 12) resemble those of tridentate pedicellarie (Pl. 
2, fig. 13) but they expand somewhat more at the tip. 
In Chetodiadema pallidum A. Ag. and Clark, eystacanths (Pl. 3) are found 
only around the abactinal system, mainly upon the interambulacral plates. 
They apparently vary far more in shape than in the species of Colobocentrotus 
and Podophora. The cystacanths of Pl. 3, figs. 5, 6, 7,8 are somewhat club 
shaped like those of P. atrata. They show well developed pores (PI. 3, figs. 2, 
3,5, 7, 11). Others (Pl. 3, fig. 1) look more like the wrinkled sacs of the 
cystacanths of Colob. Stimpsoni. Others again consist only of a single 
elongated sac (Pl. 3, figs. 2, 9). Transparent bladders like those seen in 
P. pedifera are seen on Pl. 3, figs. 1, 2, 11. It will be interesting to see if 
cystacanths are found in other echini and to obtain, if possible, some clue 
to the function of these problematical organs which suggest affinities both 
to the poison glands of globiferous pedicellariz and to the sac-carrying spines 
of Echinothuriae. 
On Pl. 12, figs. 8-10 of the Ingolf Ex. Echini, Dr. Mortensen gives sections 
of an organ which remind one of cystacanths, but he calls them globiferous 
pedicellariz, though they seem to be without valves or other calcareous deposits. 
He gives no further explanation of their structure in describing the glob- 
iferous pedicellaria of Haplos. pellucidum. 
Cystacanths vary greatly in number in different specimens, and their occur- 
