380 THE ANIMAL KINGDOM 



Figure 19.19. Echinoderm larvae. Upper right, ventral view of an auricularia of 

 the sea cucumber, Labidoplax digitata (compare with Fig. 19.18 F). Upper left, pluteus 

 larva of the sea urchin, Psamniechinus miliaris. Lower, pluteus larva of the brittle star, 

 Ophiothrix fragilis. (Courtesy Douglas P. Wilson.) 



preoral lobe, and may be used for temporary attachment to the ocean 

 bottom. 



The ring canal develops five branches which will become the radial 

 canals, and the body around the mouth begins to grow out in the five- 

 part radial symmetry of the adult. The pore of the anterior coelomic 

 pouch migrates to the original right side and the anterior pouch becomes 

 constricted to form two portions. The preoral lobe with its sensory tuft 

 and coelomic cavity degenerates and is absorbed, while the pore and a 

 portion of the anterior pouch become the madreporite and stone canal 

 (and associated structures) of the adult. Thus, the lower side of a starfish 

 develops from the left side of the larva, while the upper side develops 

 from the larval right side. The third pair of coelomic pouches, which 

 lie left and right in the larva, become upper and lower in the adult, with 

 a horizontal mesentery between them. All that remains of this mesentery 

 in the adult is the five pairs of retractors of the cardiac stomach. 



The steps in this metamorphosis that are general for echinoderms 

 include: (1) the development of an adult oral surface from the larval left 

 side, and an aboral surface from the larval right side; (2) the develop- 

 ment of most of the water vascular system from the left middle coelomic 



