A. II. Clark — Pentamerous Symmetry of Crinoidea. 353 



Aet. XXX. — Remarks on the Pentamerous Symmetry of the 

 Crinoidea; by Austin H. Clark. 



The pentamerous symmetry of the echinoderms is now 

 admitted to be a feature of phylogenetically secondary import- 

 ance — a radial symmetry superimposed upon a primarily bilat- 

 eral symmetry and so perfectly developed as almost entirely 

 to mask the original condition. It has been pertinently sug- 

 gested that the sedentary life of the animals has induced a 

 radial arrangement of their structures, analogous to what has 

 occurred in certain other groups, though in the echinoderms 

 this has been carried so far as to result in a complete absence 

 of any head region, and, consequently, a complete decentraliza- 

 tion of the nervous system, so that, in many cases, the only 

 trace of the original bilateral symmetry is found in the diges- 

 tive system. In the Astroradiata * (Asteroidea and Ophiu- 

 roidea) even this has yielded to the radial tendency, through 

 here the water-vascular system, as evidenced by the so-called 

 "stone-canal," shows indications of a former bilateral arrange- 

 ment. 



The larva of all echinoderms are bilaterally symmetrical, and 

 pronouncedly so, the pentamerous symmetry only appearirg 

 when the adult form is acquired. The astroradiate echino- 

 derms are all, except for the stone canal, radially symmetrical 

 in every respect; but in the heteroradiate forms (Pelmatozoa, 

 Echinoidea, and Holothuroidea) there is often, one might 

 almost say usually, a more or less pronounced leaning toward 

 the bilateral type, as seen, for instance, in the Comasteridse 

 among the crinoids, in the clypeastroids and spatangoids among 

 the urchins, and in the Elasipoda, and in such forms as 

 Lophothuria and Psoitis, among the holothurians. There are 

 two possible explanations of this bilateral condition in the 

 heteroradiate echinoderms ; (1) the animals may never have 

 attained the perfected radial symmetry, or (2) they may have 

 passed through a radially symmetrical condition and, reacquir- 

 ing the habit of moving in a definite direction, have again taken 

 on a bilateral habit, this bilateral condition having been super- 

 imposed upon the previous pentamerous condition, which itself 

 was superimposed upon the primitive bilateral state. It is 

 probable that the so-called "irregular" urchins (the clypeastroids 

 and spatangoids) as well as the Elasipoda and other bilateral 

 holothurians exemplify the latter condition, while certain of 

 the Pelmatozoa furnish an instance of the former. It is among 

 the crinoids, then, of the living echinoderms that an explana- 



* American Naturalist, vol. xliii, p. 686, Nov., 1909. 



