54 



It "will bo fonveiiieut to o'ive details iiiuler three 

 lieadiiig's : — Nuelial Ivoof, Vestigial Cteuidia, Pallial 

 Gills. 



The Roof of tlie Xkc/kiI Chomher has already been 

 mentioned, and it has 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 ^-avity to 

 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 pait 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- 

 Ijdng 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, Avhere it is also stated that Bouvier does not 

 accept as such what are here described as ctenidial 

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

 taining blood spaces, and situattMl near the osphradium ; 

 these blood spaces contain numerous corpuscles, and 

 Boutan remarks that the mass resembles a lymph gland. 



The I'^econdanj or Pallial Gills have already been 

 mentiimed. 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, 



