﻿420 
  Prof. 
  M'Intosh 
  on 
  the 
  

  

  I. 
  The 
  Cutaneous 
  System. 
  

   1. 
  The 
  Cuticle. 
  

  

  In 
  the 
  snout 
  the 
  cuticle 
  forms 
  a 
  dense 
  translucent 
  invest- 
  

   ment, 
  thickest 
  in 
  the 
  median 
  line 
  dorsally 
  and 
  diminishing 
  

   toward 
  each 
  lateral 
  margin. 
  On 
  the 
  ventral 
  surface 
  the 
  same 
  

   arrangement 
  occurs, 
  but 
  the 
  coat 
  is 
  appreciably 
  thinner. 
  In 
  

   front 
  of 
  the 
  mouth 
  the 
  depth 
  of 
  the 
  cuticle 
  increases 
  in 
  the 
  

   median 
  ventral 
  region, 
  especially 
  behind 
  the 
  separation 
  of 
  

   the 
  central 
  longitudinal 
  muscles. 
  It 
  then 
  diminishes 
  in 
  

   thickness 
  and 
  becomes 
  continuous 
  with 
  the 
  superficial 
  layer 
  

   of 
  the 
  oral 
  region, 
  and 
  will 
  be 
  considered 
  under 
  that 
  head. 
  

   Toward 
  the 
  posterior 
  end 
  of 
  the 
  snout 
  a 
  considerable 
  layer 
  

   occurs 
  on 
  the 
  body-wall 
  below 
  the 
  decreasing 
  lateral 
  expan- 
  

   sions, 
  which 
  likewise 
  have 
  a 
  thicker 
  covering 
  than 
  in 
  front. 
  

   In 
  the 
  anterior 
  region 
  of 
  the 
  body 
  it 
  forms 
  a 
  well-marked 
  

   layer, 
  though 
  it 
  is 
  proportionally 
  thinner 
  than 
  on 
  the 
  snout. 
  

   On 
  the 
  whole, 
  it 
  is 
  thinnest 
  on 
  the 
  lateral 
  regions 
  and 
  the 
  

   lamellae 
  beyond 
  the 
  pedicles, 
  while 
  it 
  attains 
  its 
  maximum 
  

   on 
  the 
  ventral 
  surface 
  under 
  the 
  nerve-cords 
  and 
  toward 
  

   the 
  median 
  line. 
  

  

  When 
  the 
  living 
  animal 
  is 
  viewed 
  from 
  the 
  ventral 
  surface 
  

   the 
  small 
  hypodermic 
  pads 
  existing 
  on 
  the 
  fifth, 
  sixth, 
  seventh, 
  

   and 
  eighth 
  segments, 
  just 
  in 
  front 
  of 
  the 
  transverse 
  division 
  

   of 
  the 
  feet, 
  cause 
  a 
  distinct 
  forward 
  arch 
  of 
  the 
  cuticle 
  in 
  

   the 
  median 
  line. 
  The 
  elevation 
  on 
  the 
  fourth 
  segment 
  is 
  

   indistinct 
  and 
  the 
  eighth 
  is 
  less 
  than 
  the 
  seventh. 
  At 
  the 
  

   latter 
  a 
  pale 
  belt 
  is 
  observed 
  on 
  each 
  side, 
  slanting 
  backward 
  

   to 
  meet 
  in 
  the 
  median 
  line, 
  and 
  from 
  the 
  point 
  of 
  junction 
  a 
  

   single 
  central 
  baud 
  is 
  continued 
  posteriorly. 
  Such 
  probably 
  

   indicates 
  the 
  union 
  of 
  the 
  neural 
  canals. 
  

  

  In 
  the 
  second 
  or 
  posterior 
  region 
  of 
  the 
  body 
  the 
  cuticle 
  

   is 
  thinner 
  than 
  in 
  the 
  first, 
  the 
  densest 
  part 
  being 
  in 
  the 
  

   ventral 
  median 
  line, 
  where 
  the 
  nerve-cords 
  occur. 
  It 
  is 
  very 
  

   attenuate 
  at 
  the 
  tip 
  of 
  the 
  tail. 
  

  

  The 
  cuticle 
  (with 
  the 
  hypoderm) 
  is 
  thrown 
  into 
  very 
  

   regular 
  wrinkles 
  in 
  contraction. 
  Pores 
  are 
  very 
  difficult 
  to 
  

   see 
  in 
  this 
  apparently 
  structureless 
  tunic, 
  even 
  in 
  the 
  most 
  

   favourable 
  regions, 
  such 
  as 
  the 
  cuticle 
  covering 
  the 
  tips 
  of 
  

   the 
  tentacles 
  aud 
  the 
  papillae 
  of 
  the 
  same 
  organs 
  ; 
  but 
  very 
  

   fine 
  specks 
  stud 
  the 
  cuticle 
  of 
  the 
  anterior 
  region 
  of 
  the 
  body. 
  

   In 
  life 
  the 
  surface 
  of 
  the 
  cuticle 
  shows 
  many 
  fine 
  palpocils. 
  

  

  It 
  is 
  this 
  coat 
  more 
  than 
  any 
  other 
  which 
  gives 
  firmness 
  

   and 
  elasticity 
  to 
  the 
  annelidan 
  surface 
  and 
  distinguishes 
  it 
  

   from 
  the 
  soft 
  ciliated 
  skin 
  of 
  the 
  Nemerteans 
  (toughness 
  of 
  

  

  