THE TWENTY-RAYED STARFISH. 215 



to Cole's finding for Astcrius forbcsi, viz., that part in proximity to 

 the madreporite is the anterior physiological end, the madreporite 

 being on the left anterior side. 



SUMMARY. 



1. The physiological anterior end in Pycnopodia helianthoides 

 corresponds to the posterior end of Ludwig's ground plan for 

 Asterias. The anterior end possesses a relatively larger number 

 of old (large) rays in early life than it does in late life. The new 

 rays are added at two interpolation zones posterior' to rays II., I. 



2. The madreporite is, as a rule, on the left anterior side ; it 

 varies in position, but it is always found on the anterior part of 

 the dorsal disk, i.e., at the base of rays III., IV., or V. The so- 

 called five primary rays (Ritter and Crocker), which, according to 

 Ludwig, correspond to 3-IV., 2-111., i-IL, 5-!., 4-V., respectively, 

 are not the posterior rays in adult life of Pycnopodia, but the ante- 

 rior rays. Ray IV. is the anterior ray in adult life. 



3. The establishment of the physiological anterior end in Pycno- 

 podia shows that the madreporic plate is anterior to the mid-dorsal 

 and the anus posterior ; this arrangement agrees fairly well with 

 Delage and Herouard's plan for the five-rayed starfish. It also 

 corresponds with Cuenot's plan for the common starfish, with Jen- 

 nings's " more or less permanent ' set/ " and with Cole's " physi- 

 ological anterior " ; it is the reverse of Ludwig's asteroid ground- 

 plan relative to the numbering of the rays. 



ACKNOWLEDGMENTS. 



The writer wishes here to express his indebtedness to Professor 

 Trevor Kincaid, of the University of Washington (Seattle), for 

 his ever-ready assistance and financial aid in the collection of the 

 data for this and the previous paper (1918) on Pycnopodia; to 

 Professors H. B. Ward and H. J. Van Cleave, of the University 

 of Illinois, for criticizing the manuscript of this contribution. 



REFERENCES CITED. 

 Agassiz, A. 



'77 North American Starfishes. Mem. Mus. Comp. Zool., Harvard Uni- 

 versity, Vol. 5 : 6365. 

 Bury, H. 



'95 The Metamorphosis of Echinoderms. Quart. Jour. Micro. Sci., Vol. 

 38: 45-138. 



