318 THE FRESH-WATER MUSSEL 



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

 (D. Gl) surrounds the stomach. The ccelome (C&l) is reduced to a 

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

 parietal (Ccel. Epthni) and visceral (Ccel. Epthn^} layers of ccelomic 

 epithelium are shown. 



The heart consists of a median ventricle ( Vent), enclosing part of the 

 intestine, and of paired auricles (Aur). 



The paired nephridia (Nphni) open by apertures into the ccelome 

 (Nph. si] and on the exterior (Nph. p). 



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

 the exterior by gonaducts (Gnd). 



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

 (C. P. Gn) 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. 



abundant oxygen and the minute organisms used as food, 

 and makes its escape at the exhalent aperture, taking with it 

 the various products of excretion and faecal matter. 



The mouth (MtJi) is anterior and ventral, lying just in 

 front of the foot : it is bounded on each side by a pair of 

 triangular bodies, the labial palpi, and leads by a short 

 gullet (Gul) into a stomach (St) from which proceeds a 

 long coiled intestine (Int) : this makes several turns in the 

 ventral region of the trunk, then passes to the dorsal region, 

 and finally backwards in the median plane to open by an 

 anus (An) at the posterior end of the body just within the 

 exhalent aperture. The enteric canal is formed almost 

 exclusively from the enteron, the stomodaeum and procto- 

 daeum being both insignificant, hence the enteric epithelium 

 is almost wholly endodermal. There is a large digestive 

 gland (D. Gl) surrounding the stomach and opening into 

 it by several ducts. 



The coelome (Ccel) is a small cavity in the dorsal region 

 containing a portion of the intestine : the rest of the enteric 

 canal is embedded in solid mesoderm. 



The mesoderm, as usual, is largely differentiated into 

 muscle, There are numerous muscles connected with the 



