NOTES ON ANOUON AND UNIO. 57 



I was very much interested to notice one morning after a severe 

 frost that the water in the dissecting-dish where an Anodon lay 

 removed from its shell was completely frozen. I allowed the frozen 

 mass to thaw gradually, and then examined the animal and its 

 Glochidia ; both were quite alive and none the worse for their 

 severe exposure. I allowed the same animal and its young to be 

 again frozen the following night, and obtained the same result. This 

 power of being frozen and recovering must be of great importance 

 in preserving the species in many of our shallower ponds and streams 

 which are frequently frozen to the bottom in severe weather. 



EXPLANATION OF PLATE I. 



Fig. 1. Diagram of Anodon to show course of ova. The left mantle-flap has 

 been reflected towards the dorsal surface and also the left gill-plates. 

 The free dorsal margin of the inner lamina of the internal gill-plate 

 has been turned up to show the surface of the nephridium (organ of 

 Bojanus). a, external nephridial aperture ; b, genital aperture ; 

 c, reflected free portion of dorsal margin of inner lamina ; d, ciliated 

 external surface of nephridium ; e, retractor pedis muscle ; /, exhalant 

 siphonal notch ; gg, probe passed through from lower to upper division 

 of subpallial chamber, passing out at/; h, oviduct. The arrows 

 indicate the direction in which the ova pass. 



2. Ventral view of shell of young Anodon, 101 days after first attachment 



to host and about 25-30 days after the end of parasitic life. The 

 Glochidium- shell is shown outside the permanent shell, and the shell- 

 teeth project inwards towards the middle line in such a way as to 

 press upon and constrict the permanent shell at a point about half- 

 way along its length. 



3. Lateral view of somewhat old Anodon. 



Figs. 4 & 5. Lateral and dorsal views respectively of left valve of small adult 

 Unio, showing the notches x, produced on each line of growth by the 

 previous constriction caused by the shell-teeth of the Glochidium-shell. 



Fig. 6. Diagram to show the direction of ciliary currents on external surface of 

 nephridium. 



7. Diagram to show valvular action of ventral edge of mantle-flaps. 



a, a', right and left valves of shell ; b, b', right and left mantle-folds ; 



c, c', thickened margins of b, b' ; d, d', lines of attachment of b, V to 

 a, a'. The arrows indicate the direction of water-pressure. 



8. Diagram of relation of gill-lamellse to show how the ova are prevented 



from falling into the internal gill, a, visceral mass ; c, mantle-flap ; 



d, axis of gill ; e, inner, er, outer lamella of external gill-plate ; /, 

 outer, fr, inner lamella of internal gill-plate ; g, line of concrescence ; i, 

 suprabranchial space of subpallial chamber. 



