EXAMINATION OF UNTO OR ANODONTA. 291 



3. The ridge formed by the union of the mner lamella (o) 

 of the outer gill (?) to the outer lamella {q) of the inner 

 gill (/«) is now attached to the walls of the body cavity, 

 thus dividing the cloacal chamber into thi-ee parallel tubes, 

 which the previous section shows to be in communication 

 with each other posteriorly. 



^lake a drawing of the section, showing all these 

 ^points. 



III. The eommination of a section through the heart. 



a. The mantle cavity is of substantially the same shape 

 las in the previous sections, but it is now di\'ided into five 

 f chambers (Fig. 148, A, i, i, k, k). 



1. Of these the branchial chamber (A) is much the 

 l^largest, and it contains not only the gills, but also the 



ibdomen, which hangs suspended over the median line of 

 le roof of the mantle cavity. 



2. The cloacal tubes {i) of the outer gills are substan- 

 [tially as in the preceding section. 



3. The median cloacal tube is now divided by the abdo- 

 len with two tubes {k, k) which may be called the cloacal 



[tubes of the inner gills. 



b. The ffills. 



1. The outer lamellae of the outer gills are still attached 

 [to the mantle, and the ridge formed b}' the union of the 



inner lamella of the outer gill to the outer lamella of the 

 [inner gill is attached to the roof of the mantle cavit3% 



2. The dorsal edge of the inner lamella of the inner 

 Igill (;«) is in this species free, so that the cloacal tube of 

 jthe inner gill is in communication with the branchial cham- 

 [ber through the branchial slit. This is also the case in 

 I Anodonta and in most of the Unionidse ; but in certain 

 [sub-genera of the genus Unio there is no such slit, and the 



ler lamella is in this region united to the intesrument of 



