62 



ESSENTIALS OF ZOOLOGY 



The fresh-water mussel inhabits the mud, the anterior end 

 being buried and the posterior end exposed. The foot enables 

 it to progress slowly by ploughing through the mud. The 

 food is suspended matter, detritus, and minute organisms. 



EXTERNAL MORPHOLOGY. Each valve has a rounded 

 anterior border and a pointed posterior border. The lines of 

 growth follow the contour of the shell and indicate the history of 

 the shell. They centre dorsally in a small rounded projection 

 which is the nucleus of growth of the shell, and it is occupied 

 also by the larval shell or by a scar which marks the position 



Anterior Digestive 



adductor gland Int. Post. add,. 



Larval shell 



Velum Mouth Gill Byssus 



gland and thread 



FIG. 27. Larva of Mya 

 arenaria with velum 

 retracted and foot pro- 

 truded. 



Foot 



FIG. 28. Young Mya arenaria, to show 

 position of the larval shell (that of 

 fig. 27) on the umbo of the adult shell. 

 After Jackson. 



of the larval shell. This prominence is styled the umbo. 

 The two valves are hinged by a straight edge, the dorsal hinge 

 line, and are connected by a ligament, which extends from a 

 point a little in front of the umbo to the end of the hinge line 

 behind it. The ligament is elastic and tends to open the 

 valves, and in the living condition it is compressed by the 

 adductor muscles. 



Internally the shell has a pearly appearance and is 

 smooth. The general internal aspect is similar to that of the 

 external surface, and the hinge line and the ligament have 

 already been seen. Along with lines which express the growth 

 of the shell certain impressions will be observed which indicate 

 the history and the attachment of muscles. These will be 

 identified as anterior adductor, anterior retractor, anterior 



