NOTES ON HISTOLOGY OF LINGULA ANATINA. 17 



For the purpose of determining how long the otocysts which 

 are present in the larvœ of the 5-15 p. c. stage persist, I searched 

 after the structures in many individuals at the identical place 

 where they are situated in the larvœ, and in all cases I could 

 discover there a pair of sharply outlined vesicles which can 

 be identified as the otocysts. The structure I was able to find 

 in our Lingula is probably one and the same organ that Morse 

 discovered twenty three years ago, since the vesicles are very con- 

 spicuous and there are no other organs which can be mistaken 

 for the otocysts. 



The otocysts are found even in the oldest individuals I have 

 examined (45 mm. in shell length). They are a pair of vesicles 

 of so conspicuous a size that an experienced observer can easily 

 see them with the naked eye. In young Lingula (4.5-10 mm. 

 in shell length) with translucent shells, we can readily detect 

 with strong reflected light a pair of otocysts under the dorsal 

 valve, the otoliths in constant dancing motion and the blood 

 corpuscles running about the otocysts. In older individuals the 

 shell increases in thickness and prevents the transmission of 

 light : in such cases we are able to detect the vesicles only in 

 fixed specimens. To have the best surface aspect of the organ 

 the material should be fixed with chromo-acetic or some other 

 mixture with chromic acid, since in a specimen thus treated 

 the connection between the valve and animal becomes loose, so 

 that the shell can easily be peeled off from the soft part without 

 danger of injuring the latter. 



With this general remark I shall enter upon a detailed 

 description of the otocysts. In a dorsal view of the fixed 

 specimen whose dorsal valve is taken off we can at once make 

 out the otocysts imbedded in the supporting substance, upon the 



