396 BULLETIN OF THE UNITED STATES FISH COMMISSION. 
at tlie will of the animal a fine stream of the secretion was thrown out and that it soon 
hardened after coming in contact with the water. The young individuals are able, in 
someway, to leave their attached byssus, wander about by means of the foot, and 
reattach themselves in new localities. 
THE MANTLE. 
If a lamellibranch be taken out of the shell, the whole animal will be seen to be 
covered by two fleshy flaps or folds (m in the figures) generally united to each other 
in the middorsal line and attached to the top or the sides of the visceral mass and 
to the adductor muscles which pierce them to become attached to the shell. Ventrally 
these folds hang down and cover the gills and foot. 
In many cases, as in JYucula , Yoldia, Area, Trigonia , Pecten, and \nomia , the 
ventral edges are free from each other and are not in concrescence with the gills. In 
the oyster ( Ostrea virginiana) the mantle folds are not connected with each other ven- 
trally, but are connected with the outer lamellae of the outer gills. In other lamelli- 
branchs the ventral borders are fused and are . connected with the gills. Solenomya 
is an exception to this class in that the mantle is fused and is not connected with 
the gills. 
My a (Figs. 23 to 31, inclusive) is a good example of the ventral concrescence of the 
mantle. In such cases there are left two small posterior apertures, so that water may 
pass in and out of the mantle chamber, and a larger anterior one for the protrusion of 
the foot. These three openings are always present in such cases, except in Solenomya. 
Here there are but two openings, a large anterior one for the foot and a single pos- 
terior opening for both exhalent and inhalent streams of water. This has also been 
noticed by Pelseneer, but he has not spoken of the method employed in separating the 
exhalent and inhalent streams. If Solenomya be put in an aquarium it gradually 
opens the valves of the shell, and the posterior opening may be seen to have the ap- 
pearance of a single slit, as represented in Fig. 61A, PI. lxxxvii, whose edges bear a 
number of tentacles. Often the sides of the slit approach each other in the center, the 
upper and lower ends assume a circular shape, and there are formed a lower inhalent 
and an upper exhalent opening (Fig. 61B). This position is quite constantly kept as 
long as the animal is undisturbed, and is similar to the condition of Anodon, where 
there is no actual concrescence between the two openings. In Anodon , however, the 
mantle edges ventrally are nowhere fused. 
In Mytilus (Figs. 33 to 41), the mantle edges lie very close to each other, but are 
not actually united. 
As has been stated, the mantle is fused to the visceral mass above or on the sides. 
In the oyster ( Ostrea virginiana ), the folds are not thus connected with the visceral 
mass at all points on both sides. They are in concrescence on the left side, which lies 
deep in the hollow of the fixed valve, but the right side is modified. Over the peri- 
cardial cavity and that portion of the visceral mass immediately anterior to it, the 
mantle is perfectly free from its dorsal border as far ventrally as the concrescence 
between it and the outer lamella of the outer gill. Back of the pericardium the mantle 
is again connected to the anterior border of the adductor. A very peculiar cavity is 
thus formed, on the right side only, and chiefly over the pericardium. It opens dor- 
sally to the exterior, and its lower border opens into the epibranchial chamber, which, 
in this region, receives water from both gills. 
