478 ANNUAL REPORT SMITHSONIAN INSTITUTION, 196 2 



A SURVIVING OEGOPHIURID 



Now, in the discussion hitherto, I have restricted the argument to 

 hard parts. It is however, possible to correlate the soft parts with the 

 skeleton and to make some inferences as to what the soft parts prob- 

 ably were like in the ancient forms. In figure 11, F-J, is shown one 

 of the most interesting results of such correlation, for the animal 

 illustrated proves to be another living member of a group of echino- 

 derms hitherto believed to have died out in the middle Paleozoic. 

 It was isolated in this way : Once it was established that Luidiidae 

 represent an ancient grade of asteroids, it became evident that the 

 original condition of the reproductive glands must have been a 

 serially paired disposition of separate gonads along the arms, not in 

 the disk as in modern forms. Further, Platasterias showed that the 

 caecum of the stomach, which extends into the arms in asteroids, is 

 also present in somasteroids, and must therefore be another ancient 

 character. Now in one surviving genus of ophiuroids, Ophiocanops 

 of Indonesia, Mortensen (1932) had already shown that serially 

 paired gonads occur in the arms, and that there is a stomach caecum 

 in each arm, as in asteroids. Also, in those ophiuroids where conspicu- 

 ous pinnate structure occurs in the skeleton, I found that the gonads 

 enter the arm. Hence there were compelling reasons for believing 

 that Ophiocanops must be a very archaic form, for it had even more 

 archaic characters than did the pinnate ophiuroids which I had 

 dissected. Could it be that this genus was wrongly classified? 



A few months ago (1962), with the kind cooperation of the Copen- 

 hagen Zoological Museum, I was enabled to dissect the skeleton of a 

 paratype of Ophiocanops. It proved to have all the characteristic 

 features of the mid-Paleozoic Oegophiuroida. There are neither 

 dorsal nor ventral arm plates (fig. 11, G, J), the madreporite lies at 

 the edge of the disk (fig. 11, I), there are no bursae and no genital 

 plates, and most parts of the skeleton can be homologized with somas- 

 teroid elements. It is therefore now apparent that the possession of a 

 stomach caecum in each ami, and serially paired branchial gonads, is 

 really part of the diagnosis of these ancient ophiuroids. The original 

 radial groove of somasteroids is still present, but is covered over by a 

 sheet of integument on the ventral side of the arm (fig. 12, A, B). 

 This must surely be regarded as most encouraging evidence that the 

 general lines of our hypothesis are fundamentally sound, for this is 

 the first time that we have been able to infer from the soft parts — I 

 stress soft parts — that a certain form must fall in a fossil assemblage, 

 itself known only from hard parts. The hard parts of Ophiocanops, 

 when dissected, provided the proofs of the whole hypothesis, for they 

 exhibit the predicted archaic condition already known from the 

 Paleozoic fossils. These data have been privately studied by my col- 



