EVOLUTION OF THE ECHINODERMS — FELL 467 



resemblance to the virgalia of Archegonaster. Now Perseyhonaster^ 

 as seen in figure 6, F, is a typical starfish, related to the well-known 

 genus Astropecten^ and differs quite considerably from any known 

 somasteroid. However, it seemed worth while to ascertain whether 

 rodlike superambulacral ossicles occur in other genera. It did not 

 take long to establish the fact that such ossicles occur only in the 

 families Astropectinidae and Luidiidae, and only in some of the 

 members of these two families do they have a rodlike form. The 

 next point I investigated was whether the skeletal ossicles ever form 

 transverse series. 



Figure 6 shows representative examples of the kind of arrangement 

 of the internal skeletal elements that occurs in various starfishes. 

 Dissections indicated that in every family except one the ossicles 

 conform to patterns such as appear in the upper diagrams, figure 6, 

 A-E. The gradients are essentially longitudinal ones, the ossicles 

 differentiating in series not coordinated transversely, but instead 

 longitudinally, retaining this arrangement throughout life. In some 

 of the Astropectinidae, as in figure 6, D, E, a tendency to transverse 

 arrangement is apparent, since the marginal longitudinal gradient 

 is nearly, though not quite, in phase with the axial gradient differ- 

 entiating the ambulacral ossicles, and the other gradients are nearly 

 coordinated also. But in the one remaining family, the Luidiidae, 

 an exact correlation occurs between all gradients, and thus the ossicles 

 differentiate not only in longitudinal gradients, but also in transverse 

 series. 



SURVIVING PLATYASTERID STARFISHES 



Figure 6, G, shows these features in Luidia^ as seen in dissection 

 from the dorsal side. It is clear that the ossicles all differentiate under 

 a double intersecting system of longitudinal and transverse gradients. 

 However, this is also a character of all known ophiuroids. Thus, the 

 Luidiidae could now be set apart from all other asteroids, and the 

 character which distinguished them was one which they shared with 

 ophiuroids. It was further very significant that Luidiidae have a 

 blind gut, with no anus, just as in ophiuroids, and have no suckers 

 on the tube feet; these two latter characters are shared by some 

 Astropectinidae, a family which (as we have just seen) also has rod- 

 like superambulacral ossicles. I was now convinced that the Luidiidae 

 must be the most ancient type of surviving asteroids, and that 

 Astropectinidae represent a transitional phase between Luidiidae and 

 other asteroids. Could it be that Luidiidae are also transitional 

 forms, linking the ancient somasteroids with asteroids? It would 

 be too long a story to relate step-by-step here, but this is an appro- 

 priate place to interpolate a later finding, in which I was assisted by 

 an idea sent to me by my colleague C. W. Wright of London, 



