62 Descriptions of Preparations. 



by Dr. Sharpey/ it is in the direction from without inwards that 

 the currents of water pass through the gills under the influence 

 of ciliary action; and it will be seen therefore that the water 

 which comes into relation with the vessels returning blood to the 

 hearty has lost as little as possible of its aerating value^ by contact 

 with other tissues of the animaFs body. It is well to add that the 

 surface of the mantle which looks towards the gills is like them 

 clothed with ciliated epithelium ; whilst its surface which looks 

 towards and secretes the two inner layers of the shelly is covered 

 with non-ciliated cylindrical cells. 



For the structure of the gills^ and the relations subsisting between 

 the vascular systems of the body generally, of the gills, and of 

 the organ of Bojanus in the Naiades, see Langer, Denkschriften 

 Akad. Wiss. Wien. viii., 1854, and xii. 1856, pp. 36, '3,'], 61 ; 

 V. Hessling, Die Perlmuscheln, pp. 316, 230, 245, 247; 

 and the figures in V. Carus' Icones Zootomicae, xix., 1,6, 8, 

 9 and ]0, which are taken from the former of these two au- 

 thorities. See also Robin, RajDport k la Societe do Biologic, 

 1852, p. 120. 



21. Fresh- WATER Mussel {Anodonta Cygnea), 



Dissected so as to show its nerve system, and the route along which the ova pass 

 from the generative gland into the interspaces 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 muscular foot has been removed on the left side to 

 show the pedal ganglion in situ ; part also of the organ of Bojanus 

 has been cut away on the same side, and the commissure which 

 connected the two inner lamellae of the two inner gills posteriorly 

 to the posterior limit of the foot has been divided, so as to show the 

 nerve cord connecting the labial with the parieto-splanchnic ganglia 

 in its entire length. The labial ganglion, which is homologous 

 with the supra-oesophageal of the Odontophorous molluscs, is seen 

 lying upon the tendon of the retractor pedis anterior, and just 



•■ Cyclopaedia of Anatomy and Physiology, Article 'Cilia,' p. 621. 



