354 THE FRESH-WATER MUSSEL LESS. 



(An) : the enteric epithelium is mostly endodermal. The digestive gland 

 (D. Gl) surrounds the stomach. The ccelome (Cat) is reduced to a 

 small dorsal chamber enclosing part of the intestine and the heart ; the 

 parietal (Ccel. Epthm) and visceral (CceL Epthrn^-) layers of coelomic 

 epithelium are shown. 



The heart consists of a median ventricle ( Vent}> enclosing part of the 

 intestine, and of paired auricles (Aitr). 



The paired nephridia (Nphm) open by apertures into the coelome 

 (Nph. st) and on the exterior (Nph. p). 



The gonads (Gon) are imbedded in the solid mesoderm, and open on 

 the exterior by gonoducts (Gnd). 



The nervous system consists of a pair of cerebro-pleural ganglia 

 (C. P. Gri) above the gullet, a pair of pedal ganglia (Pd. Gn) in the 

 foot, and a pair of visceral ganglia ( V. Gn) below the posterior adductor 

 muscle. 



the dorso-lateral region of the body of its own side, together 

 with the corresponding mantle-lobe, and it is from the epi- 

 thelium (Fig. 93, Der. Epthm) covering these parts that the 

 shell is formed as a cuticular secretion. The whole space 

 between the two mantle-lobes, containing the gills, visceral 

 mass, and foot is called the mantle-cavity. 



A single layer of epithelial cells, the deric epithelium or 

 epidermis (Der. Epthni), covers the whole external surface, 

 i.e., the body proper, both surfaces of the mantle, the gills, 

 and foot ; that of the gills and the inner surface of the 

 mantle is ciliated. Beneath the epidermis come connective 

 and muscular tissue, which occupy nearly the whole of the 

 interior of the body not taken up by the viscera, the ccelome 

 being, as we shall see, much reduced. The muscles are all 

 unstriped, and are arranged in distinct bands or sheets, 

 many of them very large and conspicuous. The largest are 

 the anterior and posterior adductors (Figs. 92, 93, and 94, 

 a. ad, p. ad\ great cylindrical muscles which pass trans- 

 versely across the body and are inserted at either end into 

 the valves of the shell, which are approximated by their 

 contraction. Two muscles of much smaller size pass from 

 the foot to the shell, which they serve to draw back : they 



