NOTES ON ANODON AND UNIO. 53 



aeration of the water and necessity of using the outer gill for 

 respiratory purposes, that normally ejection takes place singly with 

 the egg-capsules (cast off), which float off and leave the larvae in 

 masses on the bottom. I fear I am unable to endorse this account 

 in toto ; nodular ejection undoubtedly is abnormal, but ejection in 

 cords I have always found to occur in healthy individuals supplied 

 with well aerated water, aud on one occasion have seen it occur in an 

 undisturbed Anodon it its native water. It would seem that any 

 disturbance of the water irrespective of fish, if not too violent, pro- 

 vokes emission of the Glochidia in a perfectly normal manner. 



It is important to notice that the parent is able to draw back within 

 the shell the long slimy masses of Glochidia even after they have been 

 ejected a distance of 2 or 3 inches. The importance of this fact I 

 have already mentioned in dealing with the transit of ova. I 

 observed the Glochidia on several occasions, in both Anodon and Unio, 

 thus forcibly made "to enter a second time into their mother's womb." 



IV. Alleged Swimming of Glochidia. 



The belief that Glochidia can swim by clapping their valves together 

 "like Pecten or Lima" appears to be very general in this country, in 

 spite of frequent denials {e.g. Schierholz, loc. cit.). The extent of the 

 swimming-powers consists solely in "swimming to the bottom"; in 

 other words, Glochidia cannot swim. A Glochidium normally lies at 

 the bottom of the water on its dorsal surface, the ventral surface 

 being upwards and the " byssus " (so-called) streaming up into the 

 water above. In this position the Glochidium lies powerless to move 

 in any direction, and here, too, it dies unless a convenient " host " is 

 in some way brought in contact with its " byssus." If the water is 

 disturbed the Glochidia are carried about by currents, but soon fall to 

 the bottom again and are entirely unable to make headway in any 

 direction, even when they are thus temporarily suspended in mid- 

 water. 



The Glochidia are evidently peculiarly sensitive to the odour (?) of 

 fish. The tail of a recently killed Stickleback thrust into a watch- 

 glass containing Glochidia throws them all into the wildest agitation 

 for a few seconds ; the valves are violently closed and again opened 

 with astonishing rapidity for 15-25 seconds, and then the animals 



