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THE AMERICAN NATURALIST [Vol. XLVIII 



It therefore appears that the Holothurian stock branched off from 

 the Pelmatozoa before complete pentamerous symmetry of the hydro- 

 coele and associated organs had arisen, before any definite calcynal sys- 

 tem had developed, while the gonads were still a simple strand opening 

 to the exterior by a single posterior gonopore. 



The muscles used as retractors of the oesophagus were 

 other organs in which there was considerable individual 

 variation. As a general rule each of the five retractor 

 muscles consists of a single band that takes its origin from 

 the longitudinal radial mn -de about one third the way back 

 from the anterior end of the body and is inserted in front 

 into the wall of the esophageal ring. Such a retractor, 

 however, is frequently split up into several strands vary- 

 ing from two to five in number. A reference to Table III 



shows that in 76 individuals examined, 41 had retractor 

 muscles all in single bands, while 35 specimens had these 

 muscles subdivided or multiple in character. This vari- 

 ation is especially interesting when considered with 

 reference to the number of Polian vesicles. For in forty 

 cases where one Polian body was present thirty-nine bore 

 the unsplit or single retractor and there was only one 

 specimen with these muscles showing a multiple number. 

 In thirty-six cases where two or more Polian vesicles 

 were present, all but two had the retractor muscles in a 

 split or divided condition. If we consider each strand 

 as a separate retractor muscle, we may then obtain the 

 average number of retractors per individual for any 

 definite number of Polian vesicles. By a reference to 

 the fourth horizontal line of Table III, one finds that the 

 average number in individuals with one Polian vesicle i3 



