THE GENUS COLOBOCENTROTUS. 31 
Stimpsoni) would show that they are specifically identical; again such 
spheridia of Colob. Stimpsoni (Pl. 32, fig. 4, right-hand figure) and the third 
figure of Pl. 32, fig. 6 (Colob. Mertensii) should leave no doubt of their specific 
difference. 
The spheridia of Colob. Stimpson’ were taken from the left posterior and 
the left anterior ambulacra of a specimen measuring 88 mm. in diameter, and 
those of Colob. Mertensii from the odd anterior and the left anterior ambula- 
crum of a specimen measuring 77 mm. in diameter including the spines, the 
spheridia varying in length from .23 to .27 mm. 
The great variation in the shape of the spheridia and of the pedicellarie, 
depending in great measure on the position they occupy, does not seem to 
make these organs important factors in the ultimate classification of Echini, 
as has been maintained by some recent writers. At any rate, in the genera 
Colobocentrotus and Podophora we have no basis, from the data obtained by 
the examination of the pedicellaria, upon which to build the identification of 
the species of this genus. We are led to the belief that while their examina- 
tion is certainly most interesting and instructive as far as the general groups 
of Echini are concerned, the application of the special details they furnish 
can only bring about endless confusion when applied to the classification of 
the genera and species of Echini, not only to the exclusion of other characters, 
but also when given an inordinate importance in connection with other structural 
features of the group. 
While examining some specimens of P. pedifera Mr. M. Westergren 
observed in the median line of the actinal part of one of the ambulacra pecu- 
liar organs, a group of which has been figured on PI. 1, fig. 7. They might 
at first sight be taken for clusters of poison glands of some globiferous pedi- 
cellarie. These organs are small, the sacs with the spine rarely measuring 
more than one millimeter in length. Five or six are found on the actinal side 
of each ambulacrum (Pl. 1, fig. 1). They consist in P. pedifera of three spheri- 
cal sacs surrounded by an envelope (PI. 1, figs. 3, 4). Each sac has a pore 
which in the smaller organs has a more or less pentagonal shape. The larger 
sacs seem when expanded to have forced out one or more transparent blad- 
ders (Pl. 1, figs. 2-4). I could not detect any spicules or other calcareous de- 
posits in any of these diminutive organs which we may call cystacanths. 
Only one of the sacs is attached to the extremity of a minute spine, greatly 
resembling the rods of pedicellariz. 
On PI. 1 are given a number of figures showing the cystacanths of P. pedifera 
