EVOLUTION OF THE ECHINODERMS — FELL 483 



In ventral view, the arm in Platasterias exhibits transverse grooves 

 between the rows of virgalia, and the grooves are seen to be guarded 

 by rows of cover plates, lield in connective tissue webs on either side 

 of the virgalia, forming a sort of fringe. In the natural position, 

 these fringes constitute the floors of the transverse ciliary grooves. 

 But these webs are erectile. In plate 3 (1) we see a specimen in 

 which the grooves are naturally exposed, when the webs are erected. 

 The similiarity to the condition in crinoids is quite striking. In 

 figure 14, A, B, we see, above, two adjacent pinnules in a biserial 

 crinoid, in which the cover plates and webs are seen forming the food 

 groove on each pinnule, leading to the radial food groove. Biserial 

 crinoids are now extinct. They differ from other crinoids in having 

 two alternating series of brachial ossicles forming the arm. Below, 

 in figure 14, C, D, is seen the corresponding region in Platasterias^ 

 with the cover plates in the normal down position. It does look rather 

 as though the somasteroid condition must have arisen from the crinoid 

 condition by the growth of interpinnular integument, forming new 

 grooves hetiueen the pinnules, instead of on the pinnules, while the 

 cover plates and webs accordingly turned outward, so as to cover the 

 new interpinnular grooves. The opposite condition of the ambu- 

 lacral ossicles, as already argued, must have arisen from an earlier 

 alternating arrangement, as seen in some of the fossil somasteroids, 

 and as seen also in biserial crinoids. 



Figure 15 shows the relationship which may now be inferred 

 between the crinoid arm and the arm of somasteroids and asteroids. 

 Ophiuroids have been omitted only for lack of space. In figure 15, 

 A, on the left is the normal monoserial arm, as seen in most crinoids. 

 Pinnules are given off to left and right alternately. Students of 

 crinoids have already demonstrated how the next stage, the biserial 

 arm, arose from that. But this same stage (fig. 15, B) is almost 

 precisely what we arrive at if we extrapolate backward from the 

 chinianasteroid figure 15, C, using the frame of reference we have 

 now established for all Asterozoa. We may therefore infer that the 

 hypothetical "protosomasteroid" must have been a pinnulate pelmato- 

 zoan, and since crinoids are the only known pinnulate pelmatozoans, 

 the ancestor of somasteroids must be identified as some crinoid. 



Figure 14, E-G, which can only be briefly mentioned here, shows 

 how the tube-foot in Platasterias (center) can be regarded as inter- 

 mediate between the tentacle at the base of a pinnule in a young 

 crinoid, and the much larger ampullate tube-foot of Luidia. In the 

 young crinoid, when the pinnule begins to grow out, the radial water 

 vessel produces at first only one tentacle ; the others are added along 

 the pinnule later. In the asterozoans only one such structure ever 

 arises at the base of each row of virgalia. Ophiuroids have tube- feet 



672-174—63 36 



