258 J- Pi^AYFAlR McMuKRICn, 



a mesogioeal lamella mucli stroiiger thaii the rest and giving' rise 

 to secondary lamellae and it is sometimes of sufficient streiigtli to 

 sug'gest a somewliat circumscribed muscle. In niy description (1893) 

 of B. pannosa this upper lamella was so strong tliat I described 

 the sphincter as circumscribed, but further study has shown tliat 

 it is really of the diifuse variety. In studying' different sections 

 through the sphincter of B. platei 1 noticed that in some a distinctly 

 stronger process at the upper edge occurred as shown in Fig. 35, 

 but in others it was by no meaus so distinct, and a comparison of 

 the different sections showed that it was found only when the 

 section passed through the point of Insertion of a tentacle. Hertwig 

 observed the same condition in the sphincter of B. muJtipom, and 

 an examination of B. tuediae Verrill showed that the same rule 

 held good for it, while in B. pannosa, although the strong fold could 

 be detected in the intervals between the tentacles it was much 

 strenger opposite their insertions. Ou the other band I found that 

 in B. occidua a fold with lateral secondary lamellae thougli no 

 longer than the succeeding folds could be found at the upper edge 

 of the muscle opposite the insertions of the tentacles but not in the 

 intervals. 



It would seem, then, that there is a special development of the 

 sphincter whenever it lies beneath the Insertion of a tentacle and 

 that quite diiferent appearances may be presented according as a 

 section passes through such regions or between them. Probably the 

 strong Upper fold figured by Kwietniewski for B. Jcerguelensis (1896) 

 will be found to be greatly reduced in size in the intervals between 

 the tentacles, the section which he figures being apparently one 

 that passes through a tentacle, to judge from the sudden outward 

 bending of the column wall just above the muscle. Similarly it is 

 probable that the differences between Cablgren's figures of the 

 sphincter of B. muUicornis (1902) and what I find in my preparations 

 of that spccies are due to the difl:erent relations of the sections to 

 the tentacles.^) 



1) A re-examination of my prepai-ations of U. jxtiinosa has convinced 

 me that I was inistakeu in stating that the sphincter was lower on the 

 column than usual and that a true marginal sphincter was wanting. The 

 torn condition of the margin iu the specimen examined contributed to 

 the mistake and as a matter of fact the sphincter is immediately below 

 the margin , the musculature above it being the cii-cular musculature of 

 the basal portion of one of the tentacles. 



