NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—EIRK. 95 



organ. Tliis organ would not necessarily be changed in any way. 

 The loss of the stem would doubtless result in a more or less complete 

 coalescence of the lobes of the chambered organ, much as we find in 

 the case of Actinometra and Antedon. The mere subsequent fusion 

 of the infrabasals viith the centrale would probably in no wise affect 

 the organ. Certamly there would be no occasion for a transforma- 

 tion of one form into the other involving "in addition to the change 

 in orientation of the centrale, a revolution of the chambered organ, 

 and an extension or shortening (as the case might be) of the down- 

 ward prolongation of the axial nerve-cords." 



How important a part resorption of the infrabasals has played in 

 the transmutation from one form into the other must always remain a 

 moot question. It is perfectly possible that the change might be 

 effected b}^ tliis process. If in a specimen hke figure 3 the infra- 

 basals become progressively smaller and smaller it is evident that 

 before long they wdll entirely disappear and their place be taken by 

 the centrale. A very slight resorption of the proximal faces of the 

 infrabasals and a correspondmg increase in the size of the centrale 

 would result in a typical centrale of Form M. It would certainly be 

 difficult to prove in advocating this theory that in the final stages of 

 the process the renmants of the infrabasals did not fuse with the cen- 

 trale. The question is apt to remain an open one indefinitel}". 



The fusion observed m some specimens, as pointed out above, 

 between the infrabasals themselves and the infrabasals and the cen- 

 trale certainly shows that there was a tendency toward coalescence. 

 If then we have a well-marked tendency in this direction, and if aU 

 the known facts appear to warrant rather than discourage such an 

 assumption, we should feel quite justified in considering the fusion 

 theory the more probable one, with resorption playing a minor or 

 subsidiary part. 



On page 35 Springer offers three alternatives to explain the pres- 

 ence of Forms M and D in the one species : 



1. That from the eggs of either a monocyclic or dicyclic crinoid both forms were 

 indiscriminately hatched. 



2. That they hatched in one form, with a tendency in the larva to develop into the 

 other, which tendency irregularly became effective in some indi\'iduals and ineffec- 

 tive in others. 



3. That after the larval stage, by some process of addition, subtraction, or consoli- 

 dation among the hard parts of the test, a dicyclic crinoid was transformed into a 

 monocyclic or vice versa. 



From the discussion given above we may offer here an explanation 

 compounded in part from the second and third alternatives of 

 Springer. 



The larvae were certainly dicyclic and became free-swimming at 

 an early age. When a crinoid became detached from its stalk, a 



