EXAMINATION OF UNIO OR ANODONTA. 287 



mantle cavity, with which it is continuous, behind the 

 adductor muscle, so that a section of this region would 

 show a single cavity containing the gills, and open both 

 ventrally and dorsally. 



d. The sides of the mantle cavity are formed by the 

 mantle lobes (Fig. 146, a, a), each of which is made up 

 of:- 



1. An outer integument, or glandular epithelium, which 

 is normally in contact with the inside of the shell, and by 

 which the shell is excreted. 



2. An inner integument, or ciliated epithelium, which 

 faces inwards and lines the mantle cavity. 



3. A loose network of muscular fibres and connective 

 tissue, which fills the space between these two layers. 

 (The embryology of the lamellibranchs, as well as the 

 study of sections, shows that this space is a part of the 

 body cavity, which has become filled with connective 

 tissue.) 



e. If the two layers of integument be traced upwards, 

 they will be found to diverge in the upper part of the sec- 

 tion, the outer glandular layer passing over the surface of 

 the adductor muscle (Fig. 146, g), as a thin, transparent 

 pellicle (Fig. 146, b), the inner ciliated layer, on the con- 

 trary, is reflected inwards below the adductor muscle, and 

 thus forms the roof of the mantle cavity (Fig. 146, d). 



The body cavity, with its contained organs, is thus en- 

 tirely surrounded by integument. 



f. The body cavity. 



This is comparatively unimportant in this section ; it 

 contains : 



1. The adductor muscle (Fig. 146, g). 



2. The intestine (Fig. 146,^?), with its horseshoe-shaped 

 cavity and ventral ridge, which is mushroom-shaped when 

 seen in section. 



