288 L. J. THOMAS. 



type of the genus. In 1895 Murbach mentioned certain differ- 

 ences between the two forms and in consideration for his work 

 upon the Atlantic species A. G. Mayer (1901) described it as a 

 distinct species under the name of Gonionetnus murbachii. 



The terms otocyst, statocyst, and lithocyst have been used 

 interchangeably throughout the literature on Ccelenterates. In 

 this paper the term otocyst is used in referring to the entire struc- 

 ture which is supposed to function as an organ of equilibrium. 

 A careful study of the otocysts in Gonionemus has revealed the 

 fact that the terminology ordinarily employed in describing the 

 otocyst is entirely inadequate for an intelligible description of 

 the parts and understanding of their functional relations. In 

 Gonionemus the otocyst is not a simple vesicle enclosing an 

 otolith as has usually been considered the case. From a morpho- 

 logical point of view there is considerable evidence that in Gonio- 

 nemus the large vesicular structure is merely an adaptation for 

 the protection of the true sensory apparatus all of which is 

 lodged within the structure that has ordinarily been termed the 

 otolith. This obvious confusion of terms renders a detailed de- 

 scription of the organ necessary. 



Details of the organization of the otocyst are shown in Text 

 Figure I. The wall of the primary vesicle ('/>?) is the outermost 

 wall of the entire organ and encloses a fluid-filled cavity (c) 

 within which the spheroid (s) is suspended by the primary 

 pedicel (pf>~). The thick wall of the spheroid encloses the fluid- 

 filled cavity which is designated as the secondary vesicle (si'}. 

 Within the secondary vesicle rests the otolith (o), free to move 

 about within the fluid-filled chamber. Extending from the distal 

 end of the primary pedicel to the membranous lining of the 

 secondary vesicle is a distinctly differentiated region to which the 

 term secondary pedicel (sp) has been applied. 



This investigation was carried on under the general direction 

 of Dr. H. J. Van Cleave, to whom the writer is greatly indebted 

 for suggestions and for securing the material and the identifica- 

 tion of the species under consideration. Individuals of Gonio- 

 nemus murbachii were obtained through the Supply Department 

 of the Woods Hole Marine Biological Station for comparison 



