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 trabeculæ 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 trabeculæ
of Ammocoetes; similarity of position, of structure, of origin in
fibrous tissue--Summary 119

CHAPTER IV

THE EVIDENCE OF THE RESPIRATORY APPARATUS

Branchiæ 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