246 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



It will be convenient to give details under three 

 headings: — Nuchal Roof, Vestigial Ctenidia, Pallial 

 Gills. 



Tlie Roof of the Nuchal Chamber lias already been 

 mentioned, and it lias been stated to be permeated by a 

 network of blood channels; its histological structure has 

 also been discussed. The abundance of blood channels, 

 the general structure, and the fact that blood goes direct 

 from it to the auricle makes it probable that this tissue 

 acts as a respiratory organ. Probably the movements 

 of the head, nuchal floor, etc., enable the cavity in 

 function as an imperfect sort of lung when the animal 

 is left uncovered by the tide. Individuals living far up 

 the shore are uncovered for the greater part of the time, 

 and such a specialisation would undoubtedly be 

 advantageous. 



The supposed Ctenidial Vestiges are probably entirely 

 functionless, but there is a large osphradium, with under- 

 lying osphradial ganglion, in connection with each. It 

 is generally thought that the function of an osphradium 

 is the qualitative testing of the respiratory medium, and 

 the retention of these organs in Patella is an argument in 

 favour of the respiratory activity of the nuchal chamber. 

 The osphradium has been described in the account of the 

 sense organs, where it is also stated that Bouvier does not 

 accept as such what arc here described as ctenidial 

 vestiges. The vestige is a mass of 'connective tissue con- 

 taining blood spaces, and situated near the osphradium; 

 these blood spaces contain numerous corpuscles, and 

 Boutan remarks that the mass resembles a lymph gland. 



The Secondary or Pallia! GUIs have already been 

 mentioned. Each gill is triangular in form, with the 

 base attached to the mantle. The inner side is curved and 

 runs from the mantle to the outwardly projecting apex, 



