58 COMPARATIVE ANATOMY 



of a deep groove into which the mouth opens in the median 

 line just below the angle formed by the foot and anterior 

 adductor muscle. The anus lies in the mid-line posteriorly, 

 just above and behind the posterior adductor muscle. The 

 mouth is a simple round aperture destitute of jaws or any other 

 masticatory apparatus. It leads into a fairly wide but rather 



PC. 



gccr. 



Fig. 12 



Diagrams of the anatomy of Anodonta cygnea, A. The animal lying in the 

 right valve of the shell, the left mantle-flap cut away, and the left gill 

 turned up to show the cleft c between the inner gill-plate and the foot. B. 

 Diagrammatic longitudinal section of the body to show the course of the 

 alimentary canal. C. Dissection to show the nervous system, the position 

 of the heart and pericardium, and the excretory organs. D. An enlarged 

 view of the heart, pericardium, and excretory organ (organ of Bojanus). 

 an, anus; Au, auricle; Bo, excretory organ; c, cleft between foot and 

 inner gill-plate ; /", floor of pericardium ; gl, glandular part of excretory 

 organ ; ep, np, excretory pore \g.cer, cerebral ganglion ; g.p, pedal ganglion 

 with otocyst ; g.vis, visceral ganglion pair ; Hi, heart ; int, intestine ; /./, 

 labial palps ; m, mouth ; p, genital pore (the lower reference line should 

 point to /.); PC, pericardium ; Pc.o, opening of glandular parts of excretory 

 organ into the pericardium ; R^ rectum ; st, stomach ; ur, ureter or non- 

 glandular part of the excretory organ ; V, ventricle of heart ; x, opening 

 of glandular part of excretory organ into the ureter. 



short oesophagus, and this enlarges to form a stomach in the 

 upper and anterior part of the visceraT~mass. The intestine 

 passes from the lower side of the stomach, is thrown into 

 several coils in the upper or visceral part of the foot, then 

 turns upwards, runs back in the mid-dorsal line through the 



