288 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



3. If the section has passed through the parieto- 

 splanchnic ganglia, these will be seen between the lower 

 surface of the muscle and the roof of the mantle cavity, 

 upon the middle line. 



g. The mantle cavity. 



This contains the gills, and is divided by them into two 

 chambers. 



1. The branchial chamber (Fig. 146, li), which is widely 

 open below, but is bounded at the sides by the mantle 

 lobes, and above by the gills. 



2. The cloacal chamber (Fig. 146, (Z), which is bounded 

 above by the adductor muscle ; at the sides by the mantle, 

 and below by the gills. 



h. The gills. The four gill plates (Fig. 146, e,/*), are so 

 arranged as to form a double W, which separates the bran- 

 chial from the cloacal chamber. 



1 . Note that the upper margin of the outer lamella of 

 the outer gill (/) of each side is united to the surface of 

 the mantle. It is important for a correct appreciation 

 of the homology of the mantle cavity among the lamelli- 

 branchs, to bear in mind the fact that this union of the 

 gills to the mantle is a character of secondary importance, 

 which is lacking in the young of Unio and Anodonta, and 

 in many adult lamellibranchs of other families. 



2. The inner lamellae of the inner gills (e) of the two 

 sides of the body are united to each other at cZ, but the 

 ridge thus formed is free dorsally. 



3. The inner lamella of the outer gill of each side is 

 united to the outer lamella of the inner gill, and the ridge 

 thus formed is also free dorsally, and contains a small 

 blood-vessel. 



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

 points. 



