FRESH-WATER MUSSEL. 135 



In the Unionacea an external difference of shape between the male and 

 female individual is noticeable, and this difference may be considerable. 



Structure and strength of adductor muscles. Coutance, De I'e'nergie et de la 

 structure musculaire chez les Mollusques Ace"phales, Paris, 1878. Von Ihering, 

 Z. W. Z. xxx. Suppl. 1878. Plateau, A. Z. Expt. (2) ii. 1884. On attachment of 

 muscles in Anodonta. F. Muller, Schneider's Zoologische Beitrage, i. pt. 3, 1885. 

 Action of nerves on adductors. Pawlow, Pliiger's Archiv, xxvii. 1885. 



Crystalline style. Hazay, Malacozool. Blatter, iii. 1881, p. 196. Krukenberg, 

 Vergleich. Physiol. Vortrage, ii. p. 63, 1882, Heidelberg. 



Digestive tract. Langer, Dk. Akad. Wien. viii. pt. 2, 1854. Von Hessling, 

 Die Perlmuscheln, &c., Leipzig, 1859, p. 266. Liver, Frenzel. A. M. A. xxv. 

 1885 ; physiology of liver, Krukenberg, Untersuch. aus dem Physiol. Inst. Heidel- 

 berg, ii. 1882, p. 402. 



Circulatory system. Langer, op. cit. supra. Sabatier, A. Sc. N. (6) v. 1877, 



P- 35- 



Nephridium. Griesbach, A. N. 43, 1877. De Lacaze-Duthier, A. Sc. N. (4) 

 iv. 1855. Ciliated funnels or nephrostomata. Kollman, Festschrift zur Feier des 

 3oo-jahrigen Bestehens der Universitat zu Wurzburg, 1882, Basle, p. 36. Urinary 

 products. Krukenberg, Vergleich. Physiol. Studien, i. 2, p. 14, et seq. 



Differences between male and female shells. Bronn, Klass. &c., iii. i. p. 406. 

 Hazay, Malacozool. Blatter, iii. 1881, p. 170. 



27. FRESH-WATER MUSSEL (Anodonta cygned], 



Dissected to show the nervous system and the route along which the ova pass from the genera- 

 tive gland into the interlamellar cavity of the external gills, where, as in the pouch of a marsupial 

 mammal, they are lodged, and go through certain stages of their development. 



PART of the foot has been removed on the animal's left side to show the 

 pedal ganglia in situ ; the union of the inner lamellae of the inner gills 

 behind the foot has been divided, and the glandular portion of the 

 nephridium opened to show the nerve cord uniting the left cerebro-pleural 

 to the left visceral ganglion in its entire length. 



The left cerebro-pleural ganglion is seen lying upon the tendon of the 

 retractor pedis anterior muscle, and just anteriorly to that of the protractor 

 pedis muscle. It is connected to the ganglion of the same name on the 

 right side by a commissure passing above the mouth, under which a slip 

 of blue paper has been placed. It is united by a connective which passes 

 obliquely to the left hand with the pedal ganglion of its own side. The 

 right and left pedal ganglia are united closely inter se. They lie within 

 the periphery of the ' visceral mass,' a division of the body which in these 

 Molluscs is less sharply marked off from the ' foot ' proper than in many 

 other members of the phylum. Four or five delicate nerves may be seen 

 passing off from the ganglion into the muscular portion of the foot The 

 auditory vesicle may be found appended close to the line limiting the 



