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SEA-FISHERIES LABORATORY. 435 



surrounding the orifice the excretory membrane. lie states 

 that the excretory fluid is not ejected as the result of 

 muscular contraction around the walls of the sac-like 

 ureter. There are no muscles in the walls of the ureter. 

 The outflow is of a passive character, and takes place 

 whenever the excretory orifice is opened. Although it is 

 probable that the operculum does not fit tightly enough 

 to prevent the outflow of the excretory fluid, yet it is when 

 the operculum is closed that the fluid ceases to escape. 

 TThen the operculum is drawn down closely into its 

 socket, the lips of the excretory orifice are pressed together 

 because of the contraction of the excretory membrane. 

 On the contrary, when the operculum is raised the 

 membrane is extended and the orifice opens. As shown 

 by Marchal, the movements of the operculum are under 

 the control of two muscles — an elevator muscle for 

 extending the operculum membrane, and a depressor 

 muscle for drawing the operculum down into its socket. 



Mode of Excretion. 



According to Marchal, the excretory fluid is not 

 produced by diffusion across the epithelial cells of the 

 antennary gland and bladder, but is the result of an 

 actual breaking away of a part of each of the epithelial 

 cells. An examination of the excretory fluid will show 

 that there are numerous bladder-like vesicles and also 

 cells floating in the fluid. In sections of the bladder, 

 and also in teased preparations, one can see, as stated 

 above, that the protoplasm is highly vacuolated on the 

 inner side of each epithelial cell. Moreover, in some 

 cells there is one large vacuole, or vesicle, projecting 

 into the lumen of the bladder and surrounded by an 

 extremely thin layer of protoplasm. In the interior of 

 the vesicle is a fluid sometimes containing refringent 

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