1891.] MR. O. H. LATTER ON ANODON AND UNIO. 5& 



diminished, a secondary mode of attachment becomes of all- 

 importance and is furnished no longer by the parent but by the 

 adult members of the Glochidian family, in whose neighbourhood 

 the mucus has been chiefly absorbed and who alone are provided 

 with fully developed byssus-filaments. This phenomenon is the 

 more interesting as furnishing yet another case of prolonged attach- 

 ment to the parent of the young of freshwater animals (vide SoUas, 

 " On the Origin of Freshwater Faunas," Scientific Transactions of 

 the Royal DubHn Society, vol. iv. ser. ii., 1886). 



III. Emission of Glochidia. 



The female Anodon is usually stated to retain the Glochidia 

 within the external gill-plates until fish are in the neighbourhood. 

 This is certainly not always the case, for Glochidia were frequently 

 emitted in large masses and long cords after I had gently stirred 

 the water in which the Anodons were lying. Schierholz (" Entwick. 

 der Unioniden," Denk. d. kais. Akad. d. Wiss. 1889, Iv. pp. 183-214) 

 states that nodular ejection of Glochidia is abnormal, due to imper- 

 fect 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 larvse 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, and on one occasion have seen it occur in 

 an undisturbed Anodon in its native water. It would seem that 

 any disturbance of the water irrespective of fish, if not too violent, 

 provokes emission of the Glochidia in a perfectly normal manner. 



It is important to notice that the parent is able to di-aio 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 to- 

 gether "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 

 Glochidiiim 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 vv^ater above. In this position the 

 Glochidium lies powerless to m.ove 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 



