52 OSWALD H. LATTER, M.A. 



The diagram (Plate I. fig. 8), which is a modification of Lan- 

 kester's diagram (Encycl. Brit. 9th ed., Art. " Mollusca," fig. 135 D, 

 p. 690), will make these relations clearer. 



II. The Attachment of the Glochidia to the Parent Gill-plate. 



It is well known that the epithelium of the external gill-plate 

 secretes a nutritive mucus in which the young are imbedded and 

 thus retained within the gill. This mode of attachment is, however, 

 not permanent ; for if, as is often the case, the Glochidia are retained 

 for a long time after they have attained maturity, a large number 

 escape from their egg-capsules, and the so called "byssus," becoming 

 entangled in the gill-filaments and bars of concrescence, serves to 

 secure them until they are forcibly expelled from the parent. I 

 have found that the number of Glochidia in any given parent which 

 have escaped from their egg-capsules varies with the period during 

 which they have been retained since the attainment of pre-parasitic 

 maturity. It thus appears that as the nutritive mucus is used up, 

 and its power of retaining the Glochidia within the gill is therefore 

 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 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 



