MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 
397 
This chamber may be seen in section at epc in Fig. 5, Plate lxxx, where the 
epibranchial chamber is extended ‘upward past the visceral mass, pericardium, and 
rectum, and to the exterior dorsally. In the upper part of the figure the mantle is 
represented as closely applied to the body, but in the live animal it is widely opened. 
Fig. 3, just posterior to the stomach, shows the union of the mantle and visceral mass 
near the anterior border of this cavity. 
It is probable that some of the water coming into the epibranchial chamber through 
the gill lamellse passes again to the exterior by means of this unusual path, instead of 
going posteriorly through the epibranchial and cloacal chambers. This may be the 
case, because the mantle is here loose and not applied closely to the body, and the 
channel thus afforded is directly in the line of the currents from the gill into the epi- 
branchial chamber. It probably in no way aids in the aeration of blood by bathing 
so much of the mantle wall with water, for the latter is not richly supplied with blood 
spaces, as in forms like Venus and Anodon. What may have caused this asymmetrical 
condition to appear I am unable to conjecture. 
The mantle edge is more exposed than any other part of the body between the 
valves of the shell, excepting the foot when it is extended. The protrusible foot may 
be closely contracted to the visceral mass, but the mantle edge, though it may be drawn 
away from the edge of the shell, is always capable of less retraction. Fig. 96, PI. xciv, 
shows the maximum of contraction of foot and mantle in Venus, and Fig. 97, the great 
degree to which the mantle of the oyster (me) may be contracted. On account of its 
close contact with the exterior, the mantle edge in. all forms is relatively greatly thick- 
ened, and in it have been developed sensory organs, those of touch and in a few 
instances of vision. 
This 'muscular mantle edge generally possesses three primary longitudinal folds 
seen in section in many of the figures at me. In some cases, a primary fold may become 
greatly enlarged and broken up into several secondary folds. The folds are least 
marked anteriorly. They are generally pigmented, and most deeply in the ventral 
and posterior extent of the mantle edge. The cells of certain of these folds secrete 
the horny cuticle, which is reflected outwardly over the growing edge of the shell (Fig. 
23, c). Eyes and tentacles are frequently — the latter always — present in definite folds 
(Patten, No. 15; Rawitz, No. 21). Over the outer surface of the mantle, next to the 
shell, are many gland cells, which secrete a sticky substance that becomes impreg- 
nated with lime and forms new shell layers. 
THE SIPHONAL REGION OF THE MANTLE. 
In lamellibranchs the posterior parts of the mantle lobes are variously modified to 
form separate openings for the inflow and outflow of water. In certain cases, where 
the two mantle folds are free from one another, though generally opposed throughout 
most of their length, they spread apart posteriorly in two regions close together, mak- 
ing a lower inhalent and an upper exhalent opening. This is very conspicuously shown 
in TJnio or Anodon. A similar arrangement has already been described for Solenomya. 
These openings are guarded by greatly developed tentacles, which are also generally 
present at the ends of the siphons. Prof. Brooks has described (No. 2) an enormously 
developed unpaired tentacle situated on the mantle edge of Yoldia, on the right side, 
and near the base of the siphons. This sense organ, supplied with an axial nerve, 
may be extended out beyond the ends of the siphons. 
