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THE AMERICAN NATURALIST [Vol. XLVIII 



17 in the right ventral, and only 4 in the right dorsal 

 space. That is, the total number on the left side com- 

 pared with the total number on the right side bears the 

 ratio of 110 to 21. Not only is there this tendency for 

 the vesicles to be more abundant on the left side of 

 Thyone, but the totals show that the chances of a given 

 Thyone having a Polian vesicle in any given interradial 

 space decreases in a counter-clockwise direction, begin- 

 ning with the left dorsal interradial position. Coincid- 

 ing with the number of individuals examined, the maxi- 

 mum number of chances is found in the mid-dorsal inter- 

 radius, where the stone canal is always present. That is, 

 the stone canal with its madreporite is a more funda- 

 mental and stable structure than each or all of the 



The conditions are none the less interesting when we 

 compare the Polian vesicles with reference to size and 

 location, as will be seen from the examination of Table 

 II. The Polian vesicles are here divided arbitrarily 

 into three groups, designated as large, medium and 

 small, and their respective locations are shown. In addi- 



tion some Thyone had the rudiments of other vescicles, 

 each too small to be considered a distinct pouch. These 

 are designated in the table as a "rudiment." It will be 

 noticed that all of the large, and most of the medium- 

 sized vesicles are on the left side; that all the small 

 ones, and most of the rudimentary ones are on the right 

 side. The table as a whole shows that not only does the 

 number of Polian vesicles diminish in a counter-clockwise 



