176 T'HIRTY-FIFTH REPORT ON THE StaTE Museum. 
and .35 mm. in diameter, consisting generally of a hollow tube, largest at 
the base and gradually diminishing in size to the end; occasionally they 
are forked at the end, They are covered with cilia or microscopical fila- 
ments, which keep up a rhythmical motion, causing currents of 
water to constantly enter the branchial chamber. The two lobes of the 
mantle are not joined below the inhalent siphon. The lower por- 
tion of the exhalent siphon is formed by the uniting of the gills; 
the upper portion by the uniting of the two lobes of the mantle. 
On the dorsal part of the animal is an essentially oval space, which is 
the pericardial cavity. The portion of the mantle over this space is ex- 
tremely thin and quite transparent. The rest of the mantle to the 
pallial line is thicker, but still semi-transparent ; from the pallial line 
to the free edge, the mantle is considerably thickened ; at the edge it 
is very much thickened, and is divided into two portions or lips, be- 
tween which is a smaller ridge. The epidermis is deposited by that 
portion of the mantle consisting of this ridge, a fringe of the epider- 
matic layer being usually found adhering to it. The prismatic layers 
are deposited by that portion of the mantle nearest the edge; the 
nacreous layers by the other portion of the mantle. Numerous blood 
vessels extend through the mantle, frequently branching and anasto- 
mosing (PI. 5, v.e.). Below the pallial line, in the thickened portion 
of the mantle, are numerous radiating muscular fibers, more plainly 
apparent on the posterior portion (Pl. 5, m. f.). 
The mantle is also supplied with nerves proceeding from the cerebral 
and posterior ganglia (Pl. 5,a. m. n., p.m. n.) which will be spoken of 
more fully under the head of the nervous system. 
The mantle is composed of two layers, the inner one consisting of 
ciliated epithelium cells, and the outer and shell-producing layer con- 
sisting of non-ciliated cylindrical cells. 
THE CILIA. 
(Plate 7, fig. 1.) 
The cilia are extremely minute and delicate hair-like processes, varying 
in length from zg455 to zy Of an inch. The name is derived from 
the Latin word ciliwm, an eyelash. The cilia, during life, and some- 
time after death, keep up a constant, rapid, regular motion. Water 
or any fine substance coming in contact with them is rapidly propelled 
in the direction of the movement of the cilia. 
Cilia occur not only in mollusca, but also in mammals, birds, rep- 
tiles, actinia, echinoderms, etc. The motion of the cilia does not 
cease with the death of the animal, but if the parts are kept moist 
the motion will continue for some time afterward. In examining the 
