412 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



surround the entire adductor muscle and visceral mass. In Ostrea, each of the four 

 gills is connected above with the ones next to it, and the outer ones are also connected 

 with the mantle. A vertical section (Fig. 2, PL lxxix) shows each gill to be made of 

 two lamellae {ol and il), leaving a space {w) between them. They are united to each 

 other above, the top of the inner lamella of one to the top of the outer lamella of 

 the next, and on the median line the opposed inner lamellpe. In Fig. 2, which is a 

 section near the anterior end of the gills, they are also all united above to the visceral 

 mass. A little farther back, however (Fig. 4), the two inner lamellae of the inner gills 

 only are attached to the body above. Farther back still, under the adductor (Fig. 8), 

 the gills are entirely fi^ee from the parts above. In all regions, however, the outer 

 lamellae of the outer gills are united with the mantle, as may be seen in all the figures 

 of cross- sections of this form. 



By this concrescence of the gills above, the mantle chamber below is completely 

 shut off fi-om all the spaces which appear in section above them. Posteriorly these 

 epibranchial chambers open into one another in one large cavity (Fig. 8, c), forming a 

 cloacal chamber. Its position on the posterior side of the adductor muscle is shown 

 at c (Fig. 97, PI. xciv). 



The gill lamellae are made of innumerable parallel filaments united to each other 

 in various ways in different forms, and always leaving openings through which water 

 may enter the epibranchial fi*om the branchial chamber. This current is caused by 

 the ciliated cells of the filaments. The spaces between the lamellae are called water 

 tubes. The currents from the epibranchial chanibers pass posteriorly to the cloaca. 

 Into it, from above, opens the rectum in all cases (Fig. 97, r). In those forms which 

 possess siphons, the cloaca opens into the anal siphon (Figs. 93 and 94). 



The more common xjosition of the gills differs from that of the oyster, in that a 

 foot is generally present, situated on the ventral side of the visceral mass and pro- 

 truding between the inner gills. If this should occur in Ostrea and the gills should 

 then be moved up on the sides of the body, we would have the condition in Venus 

 represented in Fig. 14. The whole foot and visceral mass here separate the chambers 

 of the right and left sides, instead of their being side by side, as in the former case. 

 Behind the visceral mass and foot, the inner gills join one another, forming a branchial 

 septum, which is continued posteriorly to the base of the siphons, and still preserves 

 a complete separation between the cloaca and branchial chamber (Fig. 93, PI. xciii). 



Here also, as in Mya and many other forms, the epibranchial chamber is divided 

 anteriorly into four parts, two on each side of the body, farther back into two, and 

 finally these unite into one, the cloaca. 



In Mytilus there are no sharply defined epibranchial chambers, for, as may be seen 

 in Figs. 32 to 41, the inner lamellae of the gills do not fasten to the body wall, nor do 

 the outer lamellae unite with the mantle as they do in the forms just noticed. It thus 

 hapi^ens tliat the water tubes {ir) open directly into the branchial chamber, whence 

 their supply is obtained. The backward current is, however, confined to the dorsal 

 part of the branchial chamber and leaves it by a special siphonal opening at its pos- 

 terior extremity. (Fig. 87, PI. xciii, co, a view of the mantle edge in the posterior 

 region.) 



Filamentous gills of this sort, often, however, undergoing great complications, are 

 possessed by the great majority of lamellibranchs. The gills of a few primitive forms 

 {Nucula, Yoldia, Solenomya), however, are entirely different in appearance. They were 



