. Outlines of zoology. Zoology. AMPHINEURA. 371 paired cords the pedals are connected by numerous cross- commissures, and the viscerals or pallials are united posteriorly by a commissure above the rectum. The bilateral symmetry is shown internally, in the paired nephridia, auricles, and genital ducts. The class is of ancient origin, dating from the Silurian. There are two orders — Polyplacophora, Chiton, and Aplacophora, Neomenia. 1st Order of Amphineura. Polyplacophora (Chitonidse). The members of this order, represented on British coasts by several species of Chiton, are slugg
. Outlines of zoology. Zoology. AMPHINEURA. 371 paired cords the pedals are connected by numerous cross- commissures, and the viscerals or pallials are united posteriorly by a commissure above the rectum. The bilateral symmetry is shown internally, in the paired nephridia, auricles, and genital ducts. The class is of ancient origin, dating from the Silurian. There are two orders — Polyplacophora, Chiton, and Aplacophora, Neomenia. 1st Order of Amphineura. Polyplacophora (Chitonidse). The members of this order, represented on British coasts by several species of Chiton, are sluggish, usually vegetarian, animals, occurring. Fig. 165 —Anatomy of Chiton. A, ventral surface (after Cuvier). B, dorsal view ot alimentary canal (after Lankester). C, genital and excretory organs from dorsal surface (after Lang and Haller, diagrammatic). 711., mouth ; a., anus; br., numerous simple gills \f., foot; b., buccal mass ; /., liver ; z., intestine ; aorta ; z\, ventricle of heart; and La., right and left auricles ; ov., ovary ; od., oviduct; od'., opening of oviduct; «., part of nephridium, represented in black throughout; no., external opening of nephridium; p., outline of pericardium. from the shore to great depths. The foot is generally as long as the body ; the mantle covers the back and bears eight shell-plates (Fig. 162), perforated, in many cases at least, by numerous sensory organs, which may be in part optic ; numerous gills lie in a regular row along a groove on each side between the mantle and the foot. In most cases the eight shell-plates are jointed on one another, and the animal can roll itself up. The uncovered parts of the mantle bear spicules. Ganglia, in the strict sense, are scarcely developed, but there is a supra-cesophageal ganglionic commissure from which the visceral and pedal cords extend backwards along the whole length of the body. There are no special sense organs on the head, which is but slightly differentiated; but the pal
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
