REVISION OF PALEOZOIC STELLEROIDEA. 33 



ever, some of these plates are at times retained in the interbrachial 

 areas as large pieces, and in many forms they are still to be made 

 out clearly on the sides of the growing distal ends of the rays. Here 

 the inframargmals occur as columns of tmy closely adjoining ossicles 

 situated directly beside the adambulacrals, and in a few cases the 

 whole five primary dorsal columns (one radial, two supramarginal, and 

 two inframargmal) can be made out wdthout any accessory pieces be- 

 tween them. In other words, a Paleozoic cryptozonian may retain 

 the marginals tlu'oughout life, but because of their small size and 

 isolation one from another by accessory pieces or because of intense 

 spicuhzation, they are no longer recognizable as such. The classifica- 

 tory value of the presence or absence of marginaha is discussed else- 

 where (under Cryptozonia), and as the mframarginals are seemmgly 

 or actually lost mdependently in a number of phyla the term is 

 here used as expressive of this condition, and not necessarily of 

 relationship. 



Spencer (1914:9) takes up the origin of the wriggling habit from 

 another point of view, that is, from a study of living Stellcroidea as 

 described by MacBride. The former states that the Hving forms can 

 be grouped into two divisions as follows : 



The graspers. — Asteroid forms in which the tube-feet are used for walking, and for 

 grasping and pulling open the bivalve shells of the mollusks upon which they usually 

 feed. The ambulacralia form an arch to take the pull. 



The wrigglers. — Ophiuroid forms in which the tube-feet have lost locomotory powers 

 and become much reduced. The animals progress by wriggling movements of the 

 arm, and the ossicles of the ambulacral groove are extensively modified for this pm-- 

 pose. The food is pushed into the mouth by the first two pairs of tube-feet, which 

 become considerably enlarged and are known as buccal tentacles. 



If we trace the history of the forms backward we find that the difference between 

 them tends to disappear. Both the ' ' graspers ' ' and the ' ' wrigglers ' ' descended from 

 a third group, which I call provisionally "the primitive Asterozoa." 



The postulated "primitive Asterozoa" above referred to are based 

 upon the earhest stages of growth of starfishes, and had the following 

 characters. The animals were attached to the ground by a well- 

 developed, flexible stalk (seen in Asterina and Asterias); the disk 

 was small compared with the five arms. In connection with the 

 water-vascular system there were two scries of ossicles, (1) a double 

 column of flooring pieces forming the sides of the ambulacrum, and (2) 

 a paired covering series as a protection to the soft structures under- 

 neath. The podia emerged between the flooring pieces and these at 

 first "were arranged not exactly opposite to each other, but shghtly 

 alternatmg, and in consequence we find that both the flooring and 

 covering plates, which arc in direct association with the podia, are 

 not exactly opposite, but arranged alternately." 



