Comparative Anatomy and Phylogeny of Cyclostomes

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Serial transverse sections of the most primitive living vertebrates, the cyclostomes(lampreys and hagfishes), were prepared and studied from viewpoints of functional anatomy ana phylogeny. In lampreys, ammocoetes larvae were also examined.The most remarkable histological features of these cyclostomes were the occurrence of large blood-sinuses, diversely differentiated cartilages, and unique red muscles. These triads especially characterize the respiratory organs. In ammocoetes, the continuous movement of the velum, located between the buccal cavity and pharynx, causes incorporation of respiratory water from the mouth. In the velum, there are large blood-sinuses, red muscles, and mucocartilage. The movement of the velum is primarily caused by the red muscles, but the blood-sinus system may be important as a kind of hydroskeleton in transforming the force of muscular contraction to the overall velar movement. In adult lampreys, respiratory water shuttles between the external gill, -pore … More and branchial sac. In adults, blood-sinuses are widely distributed in the body, but the peribranchial sinus is prominent. The contraction of the red muscles of the branchial wall may be smoothly conveyed by blood in the peribranchial sinus to the branchial sac and cause contraction of the sac and outflow of the water through the gill pore. The inflow of water may be due to elastic expansion of fibrous tissues connecting branchial arches made of characteristic alveolar cartilage. In hagfishes, large blood-sinuses occur in various regions of the body, including the velum and branchial wall. In the velum, alveolar cartilage and red muscles occur, although they are histologically different from those of lampreys.In cyclostomes, characteristically differentiated muscular and cartilage systems certainly cause the respiratory movement, but the blood-sinus system may be indispensable, as a kind of hydroskeleton, for the proper movement of the respiratory organs. Based on phylogenetic analysis, however, the occurrence of blood-sinus system could not be regarded as a synapomorphy of cyclostomes. Less