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THE

ORIGIN OF VERTEBRATES

BY

WALTER HOLBROOK GASKELL

M.A., M.D. (CANTAB.), LL.D. (EDIN. AND McGILL UNIV.); F.R.S.; FELLOW OF
TRINITY HALL AND UNIVERSITY LECTURER IN PHYSIOLOGY, CAMBRIDGE; HONORARY
FELLOW OF THE ROYAL MEDICAL AND CHIRURGICAL SOCIETY; CORRESPONDING MEMBER
OF THE IMPERIAL MILITARY ACADEMY OF MEDICINE, ST. PETERSBURG, ETC.

LONGMANS, GREEN, AND CO.

39 PATERNOSTER ROW, LONDON

NEW YORK, BOMBAY, AND CALCUTTA

1908

_All rights reserved_




CONTENTS


                                                                       PAGE
  INTRODUCTION                                                            1

  CHAPTER I

  THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM

  Theories of the origin of vertebrates--Importance of the central
  nervous system--Evolution of tissues--Evidence of Palaeontology--
  Reasons for choosing Ammocoetes rather than Amphioxus for the
  investigation of this problem--Importance of larval forms--
  Comparison of the vertebrate and arthropod central nervous
  systems--Antagonism between cephalization and alimentation--
  Life-history of lamprey, not a degenerate animal--Brain of
  Ammocoetes compared with brain of arthropod--Summary                    8

  CHAPTER II

  THE EVIDENCE OF THE ORGANS OF VISION

  Different kinds of eye--Simple and compound retinas--Upright and
  inverted retinas--Median eyes--Median or pineal eyes of Ammocoetes
  and their optic ganglia--Comparison with other median eyes--Lateral
  eyes of vertebrates compared with lateral eyes of crustaceans--
  Peculiarities of the lateral eye of the lamprey--Meaning of the
  optic diverticula--Evolution of vertebrate eyes--Summary               68

  CHAPTER III

  THE EVIDENCE OF THE SKELETON

  The bony and cartilaginous skeleton considered, not the notochord--
  Nature of the earliest cartilaginous skeleton--The mesosomatic
  skeleton of Ammocoetes; its topographical arrangement, its
  structure, its origin in muco-cartilage--The prosomatic skeleton of
  Ammocoetes; the trabeculae and parachordals, their structure, their
  origin in white fibrous tissue--The mesosomatic skeleton of Limulus
  compared with that of Ammocoetes; similarity of position, of
  structure, of origin in muco-cartilage--The prosomatic skeleton of
  Limulus; the entosternite, or plastron, compared with the trabeculae
  of Ammocoetes; similarity of position, of structure, of origin in
  fibrous tissue--Summary                                               119

  CHAPTER IV

  THE EVIDENCE OF THE RESPIRATORY APPARATUS

  Branchiae considered as internal branchial appendages--Innervation of
  branchial segments--Cranial region older than spinal--Three-root
  system of cranial nerves: dorsal, lateral, ventral--Explanation of van
  Wijhe's segments--Lateral mixed root is appendage-nerve of
  invertebrate--The branchial chamber of Ammocoetes--The branchial
  unit, not a pouch but an appendage--The origin of the branchial
  musculature--The branchial circulation--The branchial heart of the
  vertebrate--Not homologous with the systemic heart of the arthropod--
  Its formation from two longitudinal venous sinuses--Summary           148

  CHAPTER V

  THE EVIDENCE OF THE THYROID GLAND

  The value of the appendage-unit in non-branchial segments--The