﻿ANATOMY 
  AND 
  AFFINITIES 
  OF 
  THE 
  TROCHIDii;. 
  69 
  

  

  Tlie 
  remarkable 
  resemblance 
  of 
  the 
  internal 
  organisation 
  of 
  

   the 
  Trochida3; 
  more 
  especially 
  of 
  the 
  species 
  of 
  Calliostoma, 
  

   to 
  that 
  of 
  Pleurotomaria 
  is 
  of 
  considerable 
  interest 
  as 
  exem- 
  

   plifying 
  the 
  very 
  close 
  relationship 
  which 
  exists 
  between 
  these 
  

   genera. 
  There 
  is 
  very 
  great 
  similarity 
  existing 
  between 
  the 
  

   digestive, 
  excretory, 
  circulatory, 
  and 
  nervons 
  systems 
  of 
  these 
  

   two 
  types. 
  Undoubtedly 
  the 
  nervous 
  system 
  of 
  the 
  Trochida3 
  

   is 
  much 
  more 
  specialised 
  than 
  that 
  of 
  Pleurotomaria; 
  there 
  

   is 
  a 
  greater 
  tendency 
  to 
  the 
  concentration 
  of 
  nerve-cells 
  into 
  

   definite 
  ganglia, 
  and 
  the 
  close 
  approximation 
  of 
  the 
  pleural 
  

   ganglia 
  to 
  the 
  pedal 
  ganglia 
  is 
  without 
  doubt 
  a 
  speciali- 
  

   sation, 
  the 
  most 
  usual 
  condition 
  in 
  Gasteropods 
  being 
  the 
  

   approximation 
  of 
  the 
  pleurals 
  to 
  the 
  cerebrals. 
  The 
  sup- 
  

   pression 
  of 
  the 
  right 
  gill 
  in 
  the 
  Trochidae 
  is 
  of 
  little 
  importance 
  

   when 
  we 
  consider 
  that 
  in 
  Pleurotomaria 
  the 
  right 
  gill 
  begins 
  

   to 
  show 
  a 
  tendency 
  towards 
  suppression, 
  since 
  it 
  is 
  smaller 
  

   in 
  size 
  than 
  the 
  left 
  gill. 
  That 
  one 
  gill 
  has 
  been 
  entirely 
  

   suppressed 
  in 
  Trochus, 
  but 
  that 
  it 
  undoubtedly 
  existed 
  in 
  

   some 
  ancestral 
  form, 
  is 
  shown 
  by 
  the 
  presence 
  of 
  a 
  vestigial 
  

   right 
  afferent 
  branchial 
  vein 
  which 
  communicates 
  with 
  the 
  

   right 
  auricle. 
  

  

  The 
  relationship 
  of 
  the 
  two 
  kidneys 
  in 
  the 
  Diotocardia 
  and 
  

   the 
  homology 
  of 
  the 
  single 
  kidney 
  of 
  the 
  Monotocardia 
  with 
  

   either 
  one 
  or 
  other 
  of 
  these 
  has 
  led 
  to 
  considerable 
  discussion, 
  

   many 
  zoologists 
  maintaining 
  that 
  the 
  single 
  Monotocardian 
  

   kidney 
  is 
  the 
  homologue 
  of 
  the 
  leftkidney 
  or 
  papillary 
  sac 
  of 
  the 
  

   Diotocardia, 
  while 
  others 
  seek 
  to 
  homologise 
  the 
  Monoto- 
  

   cardian 
  kidney 
  with 
  the 
  right 
  one 
  of 
  the 
  Diotocardia. 
  The 
  

   former 
  view 
  is 
  the 
  more 
  generally 
  accepted, 
  and 
  is 
  based 
  on 
  

   the 
  relative 
  positions 
  of 
  the 
  kidney 
  and 
  its 
  aperture 
  with 
  

   respect 
  to 
  the 
  rectum, 
  receiving 
  additional 
  support 
  from 
  

   the 
  presence 
  in 
  the 
  Diotocardia 
  (Trochus) 
  of 
  a 
  reno-peri- 
  

   cardial 
  canal 
  placing 
  the 
  left 
  kidney 
  in 
  communication 
  with 
  

   the 
  pericardium, 
  and 
  the 
  supposed 
  absence 
  of 
  a 
  similar 
  

   structure 
  between 
  the 
  right 
  kidney 
  and 
  the 
  pericardium. 
  

   Further, 
  von 
  Erlanger's 
  researches 
  on 
  the 
  embryology 
  of 
  

   Paludina 
  (13) 
  tend 
  to 
  give 
  support 
  to 
  this 
  view. 
  Ho 
  

  

  