IJ4 ^ v - K - FISHER. 



ordinary autotoniy, the disk remaining entire with the five oral 

 angles uninjured. In fissiparity two opposite oral angles are 

 split neatly in twain. 



The location of the madreporites with reference to the plane of 

 splitting would provide two directly opposed "physiologically 

 anterior" points (Cole) and would thus automatically favor an 

 equal splitting of the disk. Crozier (20) regards the multiplica- 

 tion of madreporites at separated points on the disk of Coscin- 

 asterias tenuispina as furnishing an assurance that portions of 

 the body separated by autotomy will each be provided with a 

 madreporic canal. This seems reasonable. However, a large, 

 non-fissiparous species, Acanthaster planci, with upward of 

 sixteen rays has four to eight madreporites. 



The utility of several madreporites in fissiparous species would 

 appear to be clear. But as to origin, it is not evident in Scleras- 

 terias that the extra madreporites are solely post-larval develop- 

 ments as a preparation for fission. Furthermore we have a 

 transitory post-larval hexamerous symmetry to account for in a 

 characteristically pentamerous genus. The six-rayed young 

 with four madreporites may have descended from larvae with four 

 hydropores. If so it is likely that we have in nature the sort of 

 hydropore duplication reported by Newman in laboratory 

 cultures of Patiria miniata (Newman 21, 21 a). This physio- 

 logical twinning in the larva may be here a normal precursor to a 

 subsequent post-larval "untwinning," by which the six-rayed, 

 four-pored, fissiparous young becomes a five-rayed, non-fissi- 

 parous adult with one madreporite. [I have specimens showing 

 this last stage, before the spines and pedicellariae have assumed 

 the fully adult reduction and concentration.] In other words 

 some of the incentive to splitting may be due to a sort of physio- 

 logical duality locked up in the young with six rays. The five- 

 rayed young with one madreporite would naturally be derived 

 from larva: 1 with one hydropore. Possibly the five-rayed young 

 with two and three madreporites are descended from incompletely 

 twinned larvae; or again, they may already have accomplished 

 the reduction division without showing outward signs. 



Although the mechanics behind this curious condition are as 

 yet material for speculation, the phenomenon itself seems to 

 produce a fairly definite asexual generation following close on the 



