330 



ZOOLOGY. 



Fig. 170. ALIMENTARY CANAL AND .NERVOUS SYSTEM 

 OF ANODONTA. 



right and left, the ducts of the liver, as the sole digestive 

 gland is called ; this is a semi-compact gland which closely 

 surrounds the stomach. From the ventral side of the 

 stomach comes 

 off the in- 

 testine, which, 

 after several 

 coils in the 

 visceral mass, 

 turns dorsal- 

 wards and 

 runs nearly 

 straight (hence 

 it is here called 

 the rectum) in 

 the middle 

 line through 



the pericardium (coelom) and dorsal to the posterior adductor 

 muscle to the anus. These various names given to the 

 parts of the canal indicate only a vague analogy, not 

 homology, with the similarly named parts of a Vertebrate. 

 The whole of the alimentary canal is lined by ciliated 

 epithelium. 



6. Circulation. The heart of Anodonta consists of a 

 ventricle and two auricles : verbally, the same is true of 

 the frog, but there is no real resemblance between the two. 

 Both auricles here are alike the presence of two is merely 

 due to the bilateral symmetry of the body. The ventricle 

 wraps round the rectum a somewhat surprising position, 

 until we remember that even in Vertebrates the heart 

 arises as part of the wall of the alimentary canal. From 

 the anterior end of the ventricle comes off an " anterior 

 aorta," which runs on the dorsal side of the rectum and 

 distributes blood to all the anterior part of the body. A 

 " posterior aorta " similarly comes off from the posterior 

 end of the ventricle and runs on the ventral side of the 

 rectum. From the ultimate branches of these arteries 

 the blood passes, not into proper capillaries, but into 

 irregular spaces, whence it eventually finds its way for the 



