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 CHAPTER V THE EVIDENCE OF THE THYROID GLAND The value of the appendage-unit in non-branchial segments--The double nature of the hyoid segment--Its branchial part--Its thyroid part-- The double nature of the opercular appendage--Its branchial part--Its genital part--Unique character of the thyroid gland of Ammocoetes-- Its structure--Its openings--The nature of the thyroid segment--The uterus of the scorpion--Its glands--Comparison with the thyroid gland of Ammocoetes--Cephalic generative glands of Limulus-- Interpretation of glandular tissue filling up the brain-case of Ammocoetes--Function of thyroid gland--Relation of thyroid gland to sexual functions--Summary 185 CHAPTER VI THE EVIDENCE OF THE OLFACTORY APPARATUS Fishes divided into Amphirhinæ and Monorhinæ--Nasal tube of the lamprey--Its termination at the infundibulum--The olfactory organs of the scorpion group--The camerostome--Its formation as a tube-- Its derivation from a pair of antennæ--Its termination at the true mouth--Comparison with the olfactory tube of Ammocoetes--Origin of the nasal tube of Ammocoetes from the tube of the hypophysis-- Direct comparison of the hypophysial tube with the olfactory tube of the scorpion group--Summary 218 CHAPTER VII THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES