MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 397 



This chamber may be seen in section at e2)c 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 lamellaB 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 callable 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 figiu"es 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 sm^face 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 oj)ening. This is very conspicuously shown 

 in Unio or Anodon. A similar arrangement has already been described for Solenomya. 

 These openings are guarded by greatly develoi^ed 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. 



