Chap. 32 ECHINODERMS — FORERUNNERS OF THE VERTEBRATES 655 



do not display the five-point plan that is evident enough when they are care- 

 fully examined. 



The Starfish — An Example of the Echinoderms 



The following general description applies to the common American star- 

 fishes such as Asterias jorbesi of the eastern coast south of Maine and the 

 Pacific starfish, Pisaster ochraceus. 



Appearance. The mouth and feet of a starfish are on the down or more 

 correctly oral side upon which it rests and travels (Fig. 32.4). The rear or 

 aboral surface is up. On that side, between the bases of two of the arms is the 

 ciliated sieve plate through which water is continually drawn into the body 

 (Fig. 32.5). Its position is one of the indications of bilateral symmetry present 

 even in adult starfishes. The entire surface of the body is rough with the 

 blunt spines fastened to the units of the skeleton in the body wall. Hundreds of 

 these units, the ossicles, are set close together in the soft middle layer of cells 

 (mesoderm) that formed them (Figs. 32.6, 32.7). Covering the whole surface 

 of the body including the spines and pincers (pedicellariae) is a delicate skin 



Fig. 32.4. Starfish. A detail of the oral surface. Rows of tube feet radiate 

 from the central mouth region. Most of the tube feet are extended by the 

 pressure of the watery body fluid; some have been retracted by the strap-shaped 

 muscle within each one. The tip of each foot is enlarged by a suction disk or 

 foot hold. (Reprinted from Animals without Backbones by Ralph Buchsbaum 

 by permission of The University of Chicago Press. Copyright 1948.) 



