NO. II ECHINODERMS AS ABERRANT ARTHROPODS CLARK IJ 



tion of the mouth; in lacking the apical nervous system, which has 

 either completely disappeared or has become transformed into a 

 ventral nervous system centering in a ring or rings about the mouth ; 

 in omitting the early mid-somatic development, the development of the 

 body being inter-somatic from the first; and in having moved the 

 border between the ventral and dorsal surfaces dorsally so that all 

 the post-radial plates now lie on the ventral surface, the arms being 

 formed by an extension of the body in the plane dividing the crinoid 

 radials from the plates succeeding. 



THE SEA-URCHINS AND THE BRITTLE-STARS 

 The echinoids and ophiurans differ still more widely from the 

 crinoid type. Their larvae, except in special cases, are extraordinarily 

 developed plutei which have no attached stage at all, and are char- 

 acterized by the small size of the preoral lobe, by the great development 

 of the post-anal portion of the body, and by the possession of a special 

 larval skeleton supporting the arms which is later resorbed. 



Whereas the asteroids differ from the crinoids in transferring the 

 post-radial plates from the dorsal to the ventral surface and thereby 

 forming a ventral skeleton of primarily dorsal elements, the echinoids 

 have gone further and have eliminated the dorsal surface altogether 

 except for a ring of plates about the periproctal region and the small 

 area within it, the globular body being composed of plates representing 

 the ventral plates of the asteroids. The perfection of an entirely new 

 type of compact radially symmetrical body from the crinoid through 

 the asteroid, simulating the compact radially symmetrical coelenterate 

 body, has furnished a starting point for new development, and bilateral 

 symmetry, superposed upon the perfected radial symmetry, has reap- 

 peared and in some cases (as in Pourtalesia) has been carried to an 

 extreme. 



The ophiurans are phylogenetically parallel to the echinoids, but 

 their line of specialization is entirely different. In them the relation 

 of the dorsal to the ventral surface has remained as in the asteroids, 

 but the radial (intersomatic) extensions of the body have become 

 narrowed and consolidated into highly efficient jointed appendages 

 from which all non-essential structures have been eliminated. 



THE FEEDING HABITS OF THE ECHINODERMS AND THE 

 CRUSTACEANS 



Corresponding with the progressive specialization in their structure 

 it is interesting to note a progressive specialization in the feeding 

 habits of the echinoderms. The crinoids are plankton feeders, like the 



