Fresh-water Mussel. 65 



closed canal, by the apposition of the inner gill-lamella to the 

 visceral mass, when the ova are being extruded, and the foot and 

 visceral mass retracted and compressed by the contraction of the 

 animaFs various muscles. The third portion of this canal is con- 

 stituted, below, by the commissural floor passing between the two 

 inner gills, and, above, as in the two anterior segments, by the 

 posterior part of the organ of Bojanus. It is easy to see in this 

 preparation, how, by the contraction of the retractor pedis muscles, 

 the ova would be extruded, and the upper portion of the visceral 

 mass brought into close temporary apposition with that portion 

 of each inner gill, which, ordinarily, is separated from it by a slight 

 interspace. We can see also how under these circumstances a 

 closed canal for the transmission of the ova is formed from the 

 orifice of the generative gland up to the point at which the large 

 branchial nerves enter the gills, and beyond which the internal 

 gill passage and also the outer, open into a space which, as the anus 

 opens into it also, we may call the ^ cloaca.^ This space being but 

 small in the Naiades relatively to their ovaries, is rapidly filled by 

 the large quantities of ova which are poured into it under pressure ; 

 and, the shell being closed, there is no other path left for the ova 

 to take but the one which leads into the cavity of the external gill. 

 As these animals are dioecious, and as spermatozoa are sometimes 

 found free in the interior of the gills, to which it is plain they 

 would find access more readily when inhaled with the water for 

 respiration than to the interior of the ovaries of the female, it is 

 probable that it is not till after they reach the external branchial 

 marsupium that the ova are fertilized. The ova may be found 

 in great abundance in the external gill cavity of the Anodon during 

 the autumn and winter months ; and whilst lodged there, they go 

 through several stages of their development. 



For an account of the way in which the ova reach the external gill 

 cavity, see V. Baer, Meckel's Archiv. 1830, p. 313, Taf. vii., 

 figs. I, 2, and 4. 



For observations which V. Hessling thinks make it probable that 

 the ova of one mussel may find their way into the external 

 gill-cavity of another, see Zeitschrift fur Wissenschaflitche 

 Zoologie, i86o, p. 358. 



For a detailed account of the nervous system in the Lamelli- 



F 



