66 ART. 4. N. YATSU : 



to the 31. occlusor anterior (occ. ant.) and just dorsal to the gastro- 

 parietal band (gst. pr. bd.) and the upper cœcum of the posterior 

 dorsal lobe of the liver (psL dr. Iv.) (PL V., Fig. 80, PI. VI., 

 Figs. 81, 83, 84, and PI. VII., Fig. 111). The otocyst is in 

 some larvae almost circular, in others slightly compressed longi- 

 tudinally, and in still others nearly triangular. It measures 45-55/* 

 in diameter on an average. It is completely enclosed by thick 

 but transparent walls, which are thicker at both lateral and 

 median sides than at other points. It contains in its cavity a 

 fluid which appears light red by transmitted light. The tint may 

 be due to a refraction phenomena of light which enters the fluid 

 through the protoplasmic layer of the body. We meet with this 

 light red color in many other cases : for example, in the contents 

 of the contractile vacuoles of Protozoa, or in Noctiluca. 



During the 5 p. c. stage the otocysts remain in the state just 

 described ; that is, nothing can be found in them. But at the 

 6 p. c. stage there are found in the fluid contents of the vesicle 

 a few refractile concretions, otoliths, which are in rapid motion, 

 each particle being a little apart from others (PL V., Fig. 80). 

 At the 7-8 p. c. stage the otocyst increases in size and the 

 otoliths (PL VII., Fig. Ill) become over 40 in number, each 

 granule having also been enlarged. They are now found passively 

 grouped into a central mass which all together is in a constant 

 dancing motion. This is doubtlessly caused by cilia on the inner 

 face of the otocysts, though they were not made out either in 

 living specimens or in preparations. 



I shall here describe an anomalous case of the otocyst. In 

 a larva of the 8 p. c. stage which Mr. Hayata picked out of the 

 plankton and kindly placed at my disposal (PL VIL, Fig. 112) 

 the right otocyst wns divided into two chambers by a vertical 



