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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



Figure 15. — Inferred relationship between arms of crinoids, somasteroids, and asteroids, in 

 which dominant transverse growth gradients are gradually replaced by dominant longi- 

 tudinal gradients. A, monoserial crinoid; B, biserial crinoid; C, chinianasterid somas- 

 teroid; D, platasteriid somasteroid; E, Platanaster (early platyasterid asteroid); F, Luidia 

 (later platyasterid); G, Plutonaster (astropectinid asteroid). 



that differ little from those of crinoids. It is evident that suckers are 

 completely lacking, save in the late stages of development of special- 

 ized Asteroidea. On the other hand, in echinoids, where suckers also 

 occur, they are already present in the oldest grades of echinoids 

 known to us, and dwindle in some late secondary groups of irregu- 

 lar urchins. Thus the affinities of the asterozoan tube-foot must be 

 nearer that of Pelmatozoa than that of echinoids. 



Figure 16 compares the embryology of the disk region of astero- 

 zoans with that of crinoids. Here again there is no space to go into 

 details here, but it must suffice to state tliat essentially identical plates 

 develop in both groups, in essentially the same sequence ; but whereas 

 the crinoid proceeds to elaborate them, the asterozoan nearly always 

 discards them, or supplants them by later structures. Even in those 

 asterozoans which in the adult have a naked disk, without plates at 

 all, a complete series of calyx plates develop in the young stage, only 

 to vanish again. In the upper two rows are shown two main types 

 of calyx (with and without persistent basal plates), in each case 

 left-hand and middle examples are young crinoids, and the right-hand 

 member of each row is a comparable asterozoan. However, the calyx 

 in the third row is no crinoid at all, but the ophiuroid Ophiopyrgus, 

 of which the single known specimen now lies in the British Museum, 

 and was reexamined recently at my request by Miss Ailsa Clark, 



