. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. FISHES. 227. In the brain the separate divisions are subequal in size. The vagus nerve innervates all but the anterior pair of gill slits, and in the aquatic forms bears a large ramus lateralis distrib- uted to the sense organs of the skin. The eleventh nerve is apparently a branch of the vagus, while the twelfth is repre- sented by the first, or first and second, spinal nerves. The heart, usually far forward in position, has the sinus venosus external to the atrium, the atrium single or divided by a longitudinal septum into
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. FISHES. 227. In the brain the separate divisions are subequal in size. The vagus nerve innervates all but the anterior pair of gill slits, and in the aquatic forms bears a large ramus lateralis distrib- uted to the sense organs of the skin. The eleventh nerve is apparently a branch of the vagus, while the twelfth is repre- sented by the first, or first and second, spinal nerves. The heart, usually far forward in position, has the sinus venosus external to the atrium, the atrium single or divided by a longitudinal septum into right and left auricles, the ventricle always simple and undivided. In the lower forms the conus is large and well developed; in the teleosts it is reduced to a row of valves between the ven- tricle and the bulbus. At least one pair of aortic arches per- sists in a complete state in the adult, while some or all are permanently or temporarily (amphibia) connected with the gills. A renal portal sys- tem (p. 194) occurs; and the red blood corpuscles are large, oval, and nucleated. The func- tional kidney is the mesonephros, although rarely the proneph- ros persists. The alimentary canal is comparatively short, and either terminates in a cloaca (p. 39), or the vent is anterior to the urogenital openings. No metanephros is developed, and the ova are frequently large. The Ichthyopsida are divisible into Pisces and Amphibia. Class I. Pisces. Ichthyopsida with persistent gills; paired appendages al- most always present in the shape of fins ; median fins sup- ported by dermal rays; body usually covered with dermal scales; postcava. Eustachian tube, and stapes lacking. Nos- trils (except in dipnoi) never opening into the mouth. A Fro. 233. Relations of the conus, A, in elasmobranchs and ganoids; B, in teleosts. a, auricle; b, bulbus ar- teriosus; c, conus, reduced in jS to a circle of valves; v, Please note that these images are extracted from scanned page images that may ha
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