bro-spinal tube. The visceral tube (P) contains, as in the case of the invertebrate ani mal, the alimentary canal, the heart, and certain nervous centers belonging to the so-called sympathetic system. This nervous system and the heart are situated upon opposite sides of the alimentary canal, the sympathetic corresponding in position and in forming a double chain of ganglia with the chief nervous centers of the invertebrate; so that the cerebrospinal tube appears to be a superaddition—a something not represented in the invertebrate series. In close connection with the profound difference between the chief nerve-centers of the vertebrate aud. the invertebrate, is another remarkable structural contrast. In all the higher invertebrates, with a well-developed nervous sys tem, the latter is perforated by the gullet, so that the mouth is situated upon the same side of the body as the principal masses of the nervous system; and some of the ganglia of the latter lie in front of, and others behind the esophagus. A longitudinal section of such an animal may therefore be represented by fig. 2. A similar section of a verte brated animal shows, on the contrary, the chief center of the nervous system not to be perforated by the esophagus, the latter turning away from it, and opening upon the opposite side of the body (fig. 4)."—Op. cit., p. 60. No structures having any analogy to the chorda dorsalis, or notochord, or to the visceral arches and clefts (see SKELETON), are to be found iu the embryonic condition of any of the invertebrates.
Passing on from the developmental to the structural differences, we universally have the vertebral column and the nervous centers, consisting of brain and spinal curd; and the organs of the five senses are usually present. All possess a distinct vascular system, containing blood, with red and white corpuscles in suspension, and in all (with the soli tary known exception of the a mphioxas, or lancelet), there is a compact muscular heart of two or more cavities, and provided with valves. The breathing organ communicates with the pharynx. The alimentary canal has two apertures, usually at opposite ends of he trunk, the mouth or reception aperture never formed of modified limbs, or g vorkin horizontally, as in the articulata, but provided with two bony jaws, placed one above the other, and acting vertically.
All vertebrates possess a hepatic portal system, by which' the blood of the alimen tary canal is collected into a portal vein which ramifies through the liver. The limbs may be totally absent, or one or two pair, never more. The muscles surround the bony levers on which they act, and thus, under the influence of the will, move the limbs and other parts. The sexes are distinct.
Comparative anatomists differ in their division of the vertebrates into classes, and as to the best basis of classification. Prof. Owen, iu his Anatomy of Vertebrates, admits of only four classes, viz., fishes, reptiles, birds, and mammals; whereas Milne-Edwards, Huxley, and many of our landing authorities, separate the amphibians from the rep tiles, and assign them a class by themselves. Prof. Owen, after describing the modifica tions of the piscine, reptilian, ovian, and mammalian types, observes that the vertebrates might be binarily divided into oviparous and viviparous; into anallantoic or branchiate, and allantoic or abranchiate; into henzatothermal (Gr. haima, blood, thermos, hot) having four-chambered heart, spongy lungs, hot blood, and henzatocryal (Gr. haima, blood, cram, cold). having less perfect breathing organs, less complex heart,with cold blood ; and adopts the latter. Huxley, on the other band, after noticing the division of the vertebrates into branchiate and abranchiate, and pointing out the non-homogeneous character of the abran chiates—mammals being so sir mgly separated from birds and reptiles—suggests the removal of them to an independent position. "Thus," he observes the classes, of the verte brata are capable of being grouped into three provinces: (1) The 1cl:fur/otos (corn-. prising fishes and amphibia), defined by the presence of branchite at some period of existence, the absence of an amnion, the absence of a rudimentary development of the allantois, nucleated blood corpuscle, and a parasphenoid bone in the skull; (2) the SAURIANS, defined by the absence of branthite at all periods of existence, the pres ence of a well-developed amnion and allantois, a single occipital condyle, a complex mandibular ramus, articulated to the skull by a quadrate bone, nucleated blood corpus cles, and no parasphenoid, comprising reptiles and birds: and (3) the MAMMALS, devoid of branchim, and with an amnion and an allantois, hut with two occipital condyles, and a well-developed basi-occipital, and no parasphenoid, a simple mandibular ramus, articu lated with the squamosal, and not with the quadratum, with mammary glands, and with red non-nucleated blood corpuscles."—Op. cit. p. 74.—For further details, the reader may consult Stannius's Comparative Anatomy of the Vertebeata (in German), Wagner's Comparative Anatomy of the Vertebrata,_ translated from the German by Tulk; the works of Huxley and Owen quoted in this article; and the special departments of Cuvier's Ilegne and Blanchards's L'Organi.sation du lagne Animal, now in course of pub licatiou—a work which, if ever completed, will rival Cuvier's opus magnum.