54 



It will l)e coiiveiiKMit to give details imdei' three 

 lieadiiig-s : — Xucdial INiof, Vestigial Ctenidia, Pallial 

 Gills. 



The Roof of tJie Xitch<d C/uimho- has alieady been 

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

 network of blood channels; its histological strnctnre has 

 also been discussed. The abnndance 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 to 

 function as an imperfect sort of lung when the animal 

 is left uncovered by the tide. Individuals living tar up 

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

 and such a specialisation would undoubtedly be 

 advantageous. 



The sujjposed C'tenidud ]cdif/es are probably entirely 

 fuuctionless, 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 respirat(ny medium, and 

 the retention of these organs in PutcJla is an argument in 

 favour of the respiratcuy 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 are here described as ctenidiai 

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

 taining blood spaces, and situated near the osphradium ; 

 these blood spaces contain ]iumerous cor|)Uscles, and 

 IJoutan remarks tliat the mass resembles a lymph gland. 



The Sccondarii or Pdllinl Gills 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, 



