Dr. F. A. Bather — Notes on Yunnan Cystidea. 113 



light-reflecting matrix on the darker light-absorbing stereom, and it 

 is hard to believe that these ridges really represent channels once 

 covered by epistereom. 



Even when one's mind is clear as to the facts, it is not easy to 

 understand the origin of the channel-systems. There are two 

 features to be explained : first, the number and sinuous extension of 

 the channels ; secondly, their closure by epistereom. The simplest 

 pattern, as Professor Jaekel points out (1899, p. 413), seems but a 

 modification of a single oval peripore 1 (Hdfchen) ; when the outer 

 covering is worn away, there is indeed little difference apparent. 

 The channels connecting the two pores correspond to the moat of the 

 peripore (fig. 16). How or why did they become covered? The 

 answer to that question might also furnish the clue to their 

 multiplication. To one who, like Prof. Jaekel, believes that the 

 normal diplopore was covered (antea, p. 513), there should be no 

 particular difficulty. Now the diplopore is obviously a complete 

 unit. If it had a roof of epistereom, the development of a channel- 

 system would take place by the union of the periporal floor with the 

 roof — first between the two pores, leaving a channel on each side ; 

 then along the floor of each channel, thus splitting it into two ; and 

 so on. It is assumed by this hypothesis that the channels as they 

 multiplied would move apart, at all events in the case of the diffuse 

 pattern. This explanation is beautifully simple, but it provides no 

 motive force. A diplopore seems so finished a structure, so obviously 

 adapted to some function or other, that one cannot imagine why in 

 this limited genus it should become broken up in this way. 



Let us now suppose that the diplopore was not originally roofed 

 over. The natural interpretation then seems to be that the pore- 

 canals served to bring fluid (probably ccelomic) into osmotic 

 connection with the surrounding medium, and that the fluid passed 

 out by one canal and in by the other. The combined tube probably 

 extended as a papula, and for the base of the papula the peripore 

 afforded an attachment suggestive of retractile muscles. We have 

 seen in Sinocystis how the central region of a diplopore is raised into 

 a pustule, attaining sometimes considerable height. This implies 

 deposition of stereom in the walls of the papular tube. Excess of 

 deposition would interfere with the assumed function of the papula, 

 and might lead to the closure of the pores, as Dr. Keed believes to 

 have been the case. This calcification of the wall, with consequent 

 reduction of the osmosis, would have to be counteracted, and that can 

 be effected only by increasing the surface. Here then is the motive 

 for the multiplication and extension of the channels. In the case of 

 pustule-formation, the channels seem to have remained close round 

 the original pustule, producing the concentrated plan. The diffuse 

 plan can not have been preceded by pustule-formation, and we must 

 suppose the calcification to have taken place round the margin of the 

 peripore more rapidly than in its central area. 



1 The term " peripodium " was extended to these structures by Loven (1883, 

 Pourtalesia, p. 57), since he believed that tube-feet sprung from them as from 

 the similar structures in Echinoidea. It seems advisable to drop this use of 

 the term, along with the belief that it implies. 



DECADE VI. — VOL. VI. — NO. III. 8 



