472 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



Figure 8, F, shows the general layout of the arm skeleton, in internal 

 aspect, dissected from above. The occluded second virgalia of each 

 row form conspicuous series, the origin of all the asteroid skeletal 

 elements from virgalia being most precisely indicated, so that the 

 exact homology of every plate can be stated in terms of a code-num- 

 bering system which has been proposed for the purpose. 



Returning now to the asteroid dissections, there are some other 

 points of interest. In figure 8, D, is the youngest stage of the arm 

 in Luidia as seen in cross section near the extreme tip. Already at 

 this early stage the ambulacra! ossicles are completely erected, as in 

 all asteroids, and in contradistinction to the con-esponding section in 

 Platasterias (fig. 8, E), where the ambulacral ossicles are quite 

 horizontal. Figure 9, A, illustrates the jaws of the asteroid Per- 

 sephonaster (those of Luidia are quite similar) . In each jaw there is 

 a T-shaped plate overlying a larger pair of plates, here colored in 

 Avith black and forming the actual jaw. The large plates forming 

 the greater part of the jaw are really a displaced pair of ambulacral 

 ossicles, as will now be shown. 



ALL STARFISHES FUNDAMENTALLY PINNATE 



In figure 9, B, a general dissection of the arm base and adjacent 

 jaw is shown for PlatasteHas^ in ventral aspect. The ossicles Al, A2, 

 and A3 which make up the structure have been derived from the ad- 

 jacent ambulacral series of the anns on either side. This can be 

 demonstrated by comparing the jaw with that of the young stage of 

 the oldest and most generalized asterozoan, namely the somasteroid 

 Chinianaster of the Cambro-Ordovician (fig. 9, C, D), and that of the 

 more advanced somasteroid VillehrunaMer of the same age (fig. 

 9, E) . The puzzling T-shaped plate of Platasterias and primitive as- 

 teroids would seem to represent tegminal plates of the earlier 

 somasteroids, covered over by outward growth of the plate Al ; this 

 latter plate is already sending out a small interradial extension in 

 Villebrunaster (fig. 9, E), and evidently a further extension of the 

 process led to the condition seen in Platasterias. The jaw of Chini- 

 anaster shows some curious resemblances to the base of the arm in 

 crinoids. For example, in crinoids two of the arm-joints at the base 

 of the arm tend to fuse together, to produce a double joint, termed 

 a syzygy. The inner of the two successive elements forming the 

 syzygy is termed a hypozygal, and it carries no pinnule; the outer 

 element is called the epizygal and, unlike the hypozygal, it does carry 

 a pinnule. The pinnule of the epizygal is often larger than the others, 

 and serves as a special feeding organ, being then termed an oral pin- 

 nule. It is of great interest that the second and third ambulacral 

 joints of Chinianaster are soldered together to make a syzygy, and 

 that the hypozygal carries no pinnule, while the epizygal carries an 



