EXAMINATION OF UNIO OR ANODONTA. 291 



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

 of the outer gill (/) to the outer lamella (7) of the inner 

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

 thus dividing the cloacal chamber into three parallel tubes, 

 which the previous section shows to be in communication 

 with each other posteriorly. 



e. Make a drawing of the section, showing all these 

 points. 



III. The examination of a section through the heart. 



a. The mantle cavity is of substantially the same shape 

 as in the previous sections, but it is now divided into five 

 chambers (Fig. 148, //, i, i, k, k). 



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

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

 abdomen, which hangs suspended over the median line of 

 the roof of the mantle cavity. 



2. The cloacal tubes (?) of the outer gills are substan- 

 tially as in the preceding section. 



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

 men with two tubes (k, k) which may be called the cloacal 

 tubes of the inner gills. 



b. The gills. 



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

 to the mantle, and the ridge formed by 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 cavity. 



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

 gill (m) is in this species free, so that the cloacal tube of 

 the inner gill is in communication with the branchial cham- 

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

 Anodonta and in most of the Unionidse ; but in certain 

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

 inner lamella is in this region united to the integument of 



