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Show 1891.] MR. O. H. LATTER ON ANODON AND UNIO. 55 diminished, a secondary mode of attachment becomes ot 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 attachment to the parent of the young of freshwater animals (vide Sollas, " On the Origin of Freshwater Faunas," Scientific Transactions of the Royal Dublin 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, lv. pp. 183-214) states that nodular ejection of Glochidia is abnormal, due to imperfect 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, 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 draw back within the shell the long slimy masses of Glochidia even alter 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 |