161 



immediately after ecdysis, when the otoeyst is connected 

 with the exterior by means of a transverse slit on the 

 dorsal side of the basal segment of the antennnle. This 

 slit is quite obvious in the hard crab. The walls of the 

 sac project into the lumen at three places— (1) in the 

 postero-lateral wall (2) in the posterior wall, and (3) the 

 anterior part of the floor. 



There are three kinds of setae situated on the walls of 

 the otoeyst and projecting' into the lumen. 



The hooked setae [" Hakenkaare," Hensen] (PI. XI, 

 fig. 75) are found on the convex portion of the posterior 

 wall of the otoeyst. These setae are characterised by 

 having the distal extremity inclined at a considerable 

 angle to the proximal portion, sometimes as much as 90°. 

 The distal portion of the shaft has fine barbs arising from 

 it. The base of the shaft is sunk into a socket-like 

 depression. The total length of the hook hairs is abont 

 55//. 



The thread setae [" Fadenhaare," Hensen] are 

 found on the anterior part of the floor. They are the 

 largest setae present in the otoeyst, and are about six 

 times as long as the hook hairs. There are well-defined 

 barbs arising from the shaft. The base of each seta lies 

 in a cup-shaped depression of the chitinous wall. 



The grouped setae [' Gruppenhaare," Hensen] 

 (fig. 76) are situated on the lateral walls of the otoeyst, 

 below the closed entrance of the sac. They are about 

 140/* in length, and have blunt shafts. There are no 

 barbs present. 



The three kinds of setae are innervated from the 



otoeyst nerve, and according to Prentiss* there is one 



nerve element for each seta. The same author states that 



* Prentiss, C. W. "The Otoeyst of Decapod Crustacea: its 

 structure, development, and functions," Bull. Mtis. Comp. Zool. Harv., 

 Vol. XXXVI, No. 7. 



M 



