THE GENUS COLOBOCENTROTUS. 31 



Stimpsoni) would show that they are specifically identical ; again such 

 sphoeridia of Colob. Stimpsoni (PI. 32, fig. 4, right-hand figure) and the third 

 figure of PI. 32, fig. 6 (Colob. Mcrtensii) should leave no doubt of their specific 

 difference. 



The sphaeridia of Colob. Stimpsoni 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 7/ mm. in diameter including the spines, the O/ 

 sphoeridia varying in length from .23 to .27 mm. 



The great variation in the shape of the sphaeridia and of the pedicellariae, 

 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 pedicellarite, 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. 1. They might 

 at first sight be taken for clusters of poison glands of some globiferous pedi- 

 cellariae. 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 (PI. 1, fig. l) . They consist in P. pedifera of three spheri- 

 cal sacs surrounded by an envelope (PI. 1, figs. 3, Ji). 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 (PI. 1, figs. 2-^). 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 pedicellariae. 



On PI. 1 are given a number of figures showing the cystacanths of P. pedifera 



