206 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
the three (mal.), which projects from the lateral and posterior wall of the 
cyst, is without sensory organs of any kind. Its wall is irregularly 
curved and pitted (Plate 9, Fig. 47 mal.,), while portions of it are even 
calcified. At one point its walls are constricted to form a neck, which 
ears 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, 48, and A show the relative position of this hammer to the 
set. fil. 
set. l@. 
FIGureE A. 
Model of the lumen of the left otocyst of Carcinus, dorsal view, the upper wall of the sac 
removed. The cavity of the sac was modelled in wax from serial sections under a 
magnification of 50 diameters, and a plaster cast of the model photographed natural size. 
In making the cut this was reduced to a magnification of 33 diameters. a., anterior; 
m., median; set.’, group hairs; set. il., thread hairs; set. ta., hook hairs. 
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. ta., 
set. fil.). The smaller of these (set. ta.), located on the median portion 
