THE OTOCYST OF THE PINNID^E. 



B. H. GRAVE, 

 UNIVERSITY OF WYOMING. 



The conclusions reached in this paper were drawn from a study 

 of the otocyst in three species of lamellibranch belonging to the 

 family Pinnidse. 



I became interested in the subject some time ago, when I 

 found what appeared to be an abnormal otocyst in Atrina rigida. 1 

 In the paper referred to, it was intimated that the otocyst of 

 this lamellibranch showed signs of degeneration. Further study 

 of this organ in the same and related species has convinced me 

 that the otocyst of the Pinnidae is functionless. 



The following description was written after examining some- 

 thing over sixty specimens by the method of serial sections. 



Unlike the homologous structure of other lamellibranchs, this 

 sense organ is situated in the tip of the foot at a very considerable 

 distance from the pedal ganglion. In general, it resembles the 

 ordinary lamellibranch otocyst, but differs in being exceedingly 

 large and compound. (See Figs. I, 2 and 3.) 



Instead of being a simple capsule, it ordinarily consists of 

 several lobes, each containing an otolith. (See Fig. 3.) The 

 lobes are quite variable in size, but as a rule they are remarkably 

 large. One by measurement is 780 microns in diameter, while 

 the enclosed otolith measures 520 microns. It will be noted that 

 an object of this size can readily be seen with the unaided eye. 

 In fact the otocyst in question stands out quite prominently. 



By reference to the figures it will be noted that the larger 

 number of the lobes of these otocysts lie in contact with each 

 other. This together with the fact that the cavities of two ad- 

 joining lobes are frequently found to be in open communication, 

 indicates that they were formed from an original otocyst by 

 budding. It seems, therefore, that budding accounts for the 

 compound nature of these otocysts. 



1 B. H. Grave, "Anatomy and Physiology of Atrina rigida," Bulletin of the 

 Bureau of Fisheries, Vol. XXIX., 1909. 



