EVOLUTION OF THE ECHENODERMS — FELL 471 



Inquiries were now directed to institutions on the west coast of 

 America, requesting a search for new material. This was forth- 

 coming ahnost immediately, for by good fortune some material 

 had been taken in 1939 off Nicaragua by the Allan Hancock research 

 vessel Velero III^ though not recorded. Plate 2 shows one of the 

 splendid examples generously placed at my disposal by Capt. F. C. 

 Ziesenhenne. On the lower surface, seen in plate 2, B, the pinnule- 

 like rows of virgalia are evident even without dissection. The speci- 

 mens showed that Platasterias is able to feed on animals as large as 

 small amphipods, some of which were found wedged in the narrow 

 grooves of the arm, and in the mouth. 



Plate 3(1), also from ventral aspect, illustrates the dominant trans- 

 verse gradients of the arm and the general chinianasterid facies of 

 the animal. The soft-part characters include small tube feet without 

 suckers, a blind gut with no anus, and as expected a madreporite 

 placed at the edge of the disk, as in fossils. 



Plate 3(2) is a dissection of the skeleton, in ventral aspect, with 

 the lateral food-grooves exposed between the rows of virgalia. The 

 jaws are derived from ambulacral elements.^ A difference from 

 Chinianasteridae is seen in the establislunent of the virgalia in four 

 longitudinal rows, the homologies of which with asteroids are easy 

 to recognize. This photograph also shows some details of the radial 

 groove. The conspicuous Y-shaped processes had also been observed 

 in the fossils, though their significance was unknown. Platasterias 

 shows that a transverse adductor muscle lies between the adjacent 

 forks of tlie Y's on either side of the groove, and its action is to raise 

 the groove by approximating the ambulacrals. Thus now we can 

 understand how the fossil forms would feed, for hitherto it was puz- 

 zling that the lateral grooves seemed to lead into a flattened radial 

 region. As we now see, the groove is raised or lowered by the muscle, 

 and the fossils, being compressed, are in the lowered condition. 



Figure 8, C, shows the adductor muscle in the middle of the cross 

 section (numbered 11), in the roof of the radial groove. The struc- 

 ture of Platasterias implies ciliary currents, as suggested by the 

 arrows, and the cilia that produce them are almost certainly located 

 on the delicate microspines shown in the grooves on the right of figure 

 8, B, a ventral view. However, detailed study of wet material has not 

 yet been carried out. 



Figure 8, E, is a cross section of the arm tip in the regenerating 

 arm, the youngest stage so far investigated. The ambulacral ossicles 

 are here completely horizontal, and the outer wing is evidently grow- 

 ing between the two first virgalia, in such a manner as to wedge the 

 second one upward, thus producing the superambulacral ossicle. 



1 Not from proximal virgalia, as originally reported (Fell, 1962) 

 672-174—63 35 



