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 (PI. 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. PI. 1, figs. 1, 2 are drawn from an alcoholic 

 specimen. 



In Colob. Stivipsoni the cystacanths are comparatively larger (PI. 2, 

 figs. A-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 (PI. 2, fig. 5) a few spicules 

 could be detected. 



The cystacanths of Coloh. 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 pedicellariae (PL 



2, fig. 13) but they expand somewhat more at the tip. 



In Chcetodiadema pallidum A. Ag. and Clark, cystacanths (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 (PL 3, figs, 2, 



3, 5, 7, 11). Others (PL 3, fig. l) look more like the wrinkled sacs of the 

 cystacanths of Coloh. Stimpsoni. Others again consist only of a single 

 elongated sac (PL 3, figs. 2, 9). Transparent bladders like those seen in 

 P. jyedifera 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 pedicellariaB 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 

 pedicellariaB, though they seem to be without valves or other calcareous deposits. 

 He gives no further explanation of their structure in describing the glob- 

 iferous pedicellariae of Ilaplos. j^eUiicidum. 



Cystacanths vary greatly in number in different specimens, and their occur- 



