198 bulletin: museum of comparative zoology. 



of curvature is different for different hairs. The larger, being in a mere 

 elevated position, usually bend at a sharper angle than those near the 

 floor of the sac. All are heavily plumed ; the pinnules are long and 

 coarse (Plate 7, Fig. 31, pinn.) and often have otoliths tirmly attached 

 to them by a substance probably of glandular origin. Hensen ('63) 

 describes the otolith hairs of Crangon, as follows : " Es steht namlich auf 

 die schon erwahuten Vorbuchtung eine einzige Reihe vou 7 odei' 8 

 Haaren ; diese Haare reichen bis zur Kugel in die Steine hinein, Hire 

 Zahl erscheiyit viel zu gering fiir deren Masse. . . . Sie sind 0.075 mm. 

 lang, 0.0075 mm. breit und gerade aufgerichtet." 



This description of these hairs is completely at variance with the 

 conditions I have found in the American Crangon. In order to deter- 

 mine, thei'efore, whether this was a true specific difference, or due to an 

 error on Heusen's pai't, a number of the European specimens, procured 

 by Dr. Mark from Professor Herdman in Liverpool, were examined. 

 After dissecting out the otocysts of 12 specimens, I was entirely satisfied 

 that Hensen's description was incorrect. The hairs are precisely the 

 same in size, form, and number as in the American variety. They have 

 their shafts distinctly bent near the tip at angles varying from 25° to 90° ; 

 of the individuals examined none possessed less than twenty-four hairs 

 in the sac, the average being twenty-six. 



That Hensen should have made such a mistake is not strange. He 

 himself says: "their number appears much too small for the mass [of 

 the otoliths]." The tips of the hairs are concealed by the otoliths, and 

 only the first third of the row would be visible from above. 



d. The formation of hairs after ecdysis is identical with that of 

 Palaemonetes. 



e. The otoliths are numerous, larger than in Pala^monetes, and found 

 mostly in the posterior part of the sac, in contact with, or even attached 

 to, the fringed tips of the hairs. Mainly siliceous, they are taken in 

 after each moult, being readily pushed into the large opening of the 

 otocyst. They can be almost completely washed out by a fine jet 

 of water introduced artificially, and if the animal so treated is then 

 placed in an aquarium containing iron filings, or other substitute, this 

 material will soon be used to replace the otoliths of sand. 



2. Innervation of the Otocist. 



As in Palsemonetes, the brain is very close to the otocyst, and the 

 nerve supplying the sac is therefore short. Its general course is shown 

 at n. ot. in Figure 29 (Plate 6). 



