Limitations on Diversity of Forms: Branching of Ambulacral Systems of Echinoderms

In most well-skeletalized echinoderms of radial form, extension and branching of ambulacra occurs at or near the ends of ambulacra farthest from the mouth. In most echinoderms of compact body form, aboral coverage of ambulacra cannot be combined with the more even coverage of ambulacra which is provided by branching at the widest part of the animal. There are three ways in which echinoderms of compact body form can circumvent this limitation by a change in growth pattern: (1) freeing the water vascular system from a one-to-one correspondence with the ambulacral skeletal system, (2) adding side branches along older parts of the ambulacram, or (3) possessing separate oral and aboral ambulacral systems. The first has occurred in clypeasteroids (sand dollars) and some cystoids. The second may have occurred in some cystoids, in which ambulacral skeletal plates can slide relative to the underlying plates which support and encase the body, but has not evolved through more elaborate control of differential resorption and growth of interambulacral skeletal plates. The third appears to provide the greatest versatility with the simplest growth pattern and functional unity of skeletal and water-vascular systems, but occurs in no echinoderm, perhaps because the alteration in early development which would produce such a form has a lethal side effect (two oral sides). Thus there appears to have been no evolutionary route to this adaptive peak. Stellate forms and compact forms without aboral coverage both have the potential advantage of terminal branching of ambulacra, but stellate forms also (1) can change ambulacral distribution simply by moving the arms; (2) can alter ambulacral distribution by intercalation of arms, subterminal branching, or casting off of arms; and (3) can minimize the disadvantages associated with restriction of the ambulacral system to the oral side by reduced arm width or by bending the arms. Differences in long-term success among classes which probably had podia and lacked brachioles are consistent with the assumptions that aboral coverage by ambulacra and versatility in distributing ambulacra have both been of general adaptive importance. Blastoids were less versatile in distributing ambulacra than the cystoids; yet blastoids appeared later and became extinct later. This provides an apparent exception to Vermeij's hypothesis on increase in potential morphological versatility.