396 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



at the will of the aniinal 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 

 some way, to leave their attached byssus, wander about by means of the foot, and 

 reattach tliemselves in new localities. 



THE MANTLE. 



If a lamellibranch be taken out of the sheU, the whole animal will be seen to be 

 covered by two fleshy flaps or folds {m in the figures) generally united to each other 

 ill 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 Nucula, Yoldia, Area, Trigonia, Pecten, and Anomia, the 

 ventral edges are fi-ee from each other and are not in concrescence with the gills. In 

 the oyster {Ostrea inrginiana) the mantle folds are not connected with each other ven- 

 trally, but are connected with the outer lamelL^ of the outer gills. In other lamelli- 

 branchs the ventral borders are fased and are connected with the gills. Solenoimja 

 is an exception to this class in that the mantle is fused and is not connected with 

 the gills. 



Mya (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 iuhaleut streams of water. This has also been 

 noticed by Pelseueer, 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 lurginiana), the folds are not thus connected with the visceral 

 mass at all points on both sides. They are in concrescence on the left side, wliicli lies 

 deep in the hollow of the fixed valve, but the right side is modified. Over the peri- 

 cardial cavitj^ and that portion of the %dsceral mass immediately anterior to it, the 

 mantle is i)erfectly 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. 



