Der. 



Ccel .Epthm. 

 'CceLFpthm- 

 Sh 

 Intf 

 JJcr.Eptlnn 



FIG. 78. Diagrammatic sections of the Fresh-water Mussel. 



A, longitudinal section : the right mantle-lobe (Mant) and gills (/. G, 

 O. G) are shown in perspective. 



B, transverse section. 



The cuticular shell (Sh}, shown only in B, is black, the ectoderm 

 dotted, the nervous system finely dotted, the endoderm radially striated, 

 the mesoderm evenly shaded, and the ccelomic epithelium represented 

 by a beaded line. 



The dorsal region is produced into the right and left mantle-lobes 

 (Afant), attached to which are the valves of the shell (Sh) joined dorsally 

 by an elastic ligament (tig). 



The mantle-lobes are partly united so as to form the inhalent (Ink. 

 Ap) and exhalent (Exh. Ap) apertures at the posterior end. 



The body is produced ventrally into the foot (Foot), on each side of 

 which are the gills, an inner (/. G) and an outer (O. G), each formed 

 of an inner and an outer lamella. 



The body is covered externally by cleric epithelium (Der. Epthm), 

 within which is mesoderm (Msd) largely differentiated into muscles, of 

 which the anterior (A. Ad) and posterior (P. Ad) adductors are indi- 

 cated in A. 



The mouth (Mth) leads by the short gullet ( Gul) into the stomach 

 (St), from which proceeds the coiled intestine (//), ending in the anus 



