Lateral line organs--Function of this group of organs--Poriferous sense-organs on the appendages in Limulus--Branchial sense-organs-- Prosomatic sense-organs--Flabellum--Its structure and position-- Sense-organs of mandibles--Auditory organs of insects and arachnids-- Poriferous chordotonal organs--Balancers of Diptera--Resemblance to organs of flabellum--Racquet-organs of Galeodes--Pectens of scorpions--Large size of nerve to all these special sense-organs-- Origin of parachordals and auditory capsule--Reason why VIIth nerve passes in and out of capsule--Evidence of Ammocoetes--Intrusion of glandular mass round brain into auditory capsule--Intrusion of generative and hepatic mass round brain into base of flabellum-- Summary 355 CHAPTER XII THE REGION OF THE SPINAL CORD Difference between cranial and spinal regions--Absence of lateral root--Meristic variation--Segmentation of coelom--Segmental excretory organs--Development of nephric organs; pronephric, mesonephric, metanephric--Excretory organs of Amphioxus-- Solenocytes--Excretory organs of Branchipus and Peripatus, appendicular and somatic--Comparison of coelom of Peripatus and of vertebrate--Pronephric organs compared to coxal glands--Origin of vertebrate body-cavity (metacoele)--Segmental duct--Summary of formation of excretory organs--Origin of somatic trunk-musculature-- Atrial cavity of Amphioxus--Pleural folds--Ventral growth of pleural folds and somatic musculature--Pleural folds of Cephalaspidæ and of Trilobita--Meaning of the ductless glands-- Alteration in structure of excretory organs which have lost their duct in vertebrates and in invertebrates--Formation of lymphatic glands--Segmental coxal glands of arthropods and of vertebrates-- Origin of adrenals, pituitary body, thymus, tonsils, thyroid, and other ductless glands--Summary 385 CHAPTER XIII THE NOTOCHORD AND ALIMENTARY CANAL Relationship between notochord and gut--Position of unsegmented tube of notochord--Origin of notochord from a median groove--Its function as an accessory digestive tube--Formation of notochordal tissue in invertebrates from closed portions of the digestive tube--Digestive power of the skin of Ammocoetes--Formation of new gut in Ammocoetes at transformation--Innervation of the vertebrate gut--The three outflows of efferent nerves belonging to the organic system--The original close contiguity of the respiratory chamber to the cloaca-- The elongation of the gut--Conclusion 433 CHAPTER XIV THE PRINCIPLES OF EMBRYOLOGY