206 



BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



set.' 



the three {mal), which projects from the lateral and posterior wall of the 

 cyst, is without seusorj organs of any kind. Its wall is irregularly 

 cui'ved and pitted (Plate 9, Fig. 47 mat.,), while portions of it are even 

 calcified. At one point its walls are constricted to form a neck, which 

 b3ars a large hammer-like head (Fig. 47). This is the "Hammer" of 

 Hensen, compared by him to the malleus of the vertebrate middle ear. 

 Figures 43, 4S, and A show the relative position of this hammer to the 



set.fiL 



*el.tM. 



Figure A. 



Model of the lumen of the left otocyst of Carcinus, dorsal view, tne upper wall of the sac 

 removed. The cavity of the sac was modelled in wax from serial sections under a 

 masjiiification of 50 diameters, and a plaster cast of the model photf'^raphed natural size. 

 In making; the cut this was reduced to a mafrnitication of 'S'-i diameters, a., anterior; 

 m., median; set,', group hairs; set. fil., thread hairs; set. ta., hook tiairs. 



rest of the sac. It serves merely for the attachment of the short, thick, 

 powerful muscles of the antennule which keep the latter in almost con- 

 stant motion, and has probably nothing whatever to do with the sensory 

 functions of the otocyst. 



b. Sensory Cushions. Of the three projections noted, the remaining 

 two are sensory and bear sensory hairs (Plate 10, Fig. 55, set. to., 

 set. fil.). The smaller of these (set. ta.), located on the median portion 



