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Show 60 ON CI~ASSU'ICA'l'ION. tubes are separated by a partition which was, priinitivoly, a part of the external parietes of the bocly, but which now lioR, in a central position, between tho cerebro-spinal nervous ceutro8 and the alimentary canal. lienee, a transv rse section of any vertebrated animal may bo represented dingran1matically by Fig. 30 (III.), where, for the most part, th letters have the same sio·nifi- . n catiOn as in the foregoing cas , but where P' denotes 1he second, or cerebro-spinal, tube. TlHj vi. c ral tube (P) contains as in the case of the invertebrate anin1al, th alimentary canal: the heart, and certain nervou centres, belonging to the so-called sympathetic system. This nervous systcn1 and th heart arc situ~ted upon op~osit~ sides. ~f th ali~entary canal, the sympathetic corresponchng In position, and 1n fonuing a double chain of ganglia, with the chief n rvous centre of tho iuv rt orate; so that the cerebro- pinal tube appears t be a super-addition _a something not represented in tho in,·ert brate series. Tho formation of the cerebro-spinal tuLe of vertebrates, in tho manner which I have describ d to you, is a w li-e tablishcd fact ; nor. do I en~ertain auy doubt that the cerebro-spinal centres, VIZ., the brmn and the spinal cord of v rt Lratcs, are the ~·esult o.f a n1odification of that serou layer of the germ which Is continuous els:where wi~h th epid nni., '1\ro years ago, I took some pains to venfy the remarkable di <;overics of Remak in relation to this poh1t, and, so far a. the ehick iH concerned, his statements app r d to me to be fully borne out. But, as Von Baer long ag sugge ted, it is a nece · ·ary result of these facts that there can be no compnri 'On Letwecn the eerebro-spinal nervous centr of th Ve'f·tebrata and the ganglionated nervous centr s of the Invertebrata, and the homologues of the latter must 1 robably be souo·ht in the sym-p~~ ti~ ~ Doubtless in close connection with this profound difference ?etween the chief nervous centre f the vertebrate and the Invertebrate is another remarkaLl tructural contrast. In all the higher .invertebrate , with a well-dov loped nervous systen1, t~e latter IS perforat d by the gull t, so that the month is Situated upon the san1e side of th · body as the prineipaJ Jnasscs of the nervous . ·ystem, and so1ne of th ganglia of tho Jatter li<' 'J'H E VETITEBR A'l'A. ()1 in front of, and others behind, tho msophagn.. A longitudinal vertical section of such an animal, therefore, may be represented by Fig. 30 (II.). A similar section of a vertebrated animal shows, on the con ·trary, the chief centre of the nervous system not to be perforated by the cesophagus; the l~tter turning away from it and openiug upon tho opposite side of the body (Fig. 30, IV.). Another structure sharply distinctive of the vertebrate classes is the " chorda dorsalis " or " notochord," an organ of which no trace has yet been discovered in any of the invertebrates, though it invariably exists, in early embryonic life at least, in every vertebrate. Defore the cerebro-spinal canal is complete, in fact, the substance of the centre of its floor, beneath the primitive median line of tho embryo, becomes djfferentiated into a rod-like cellular structure, which tapers to both its extremities; and, in a histological sense, rcn1aius comparatively stationary, while the adjacent embryonic tissues are undergoing the most raJ?id and varied metamorphoses. To these great clifferences between vertebrates and invertebrates, in their early condition, many others might be added. In all Vertebrata that part of the wall of the body which lies at the sides of, nncl immediately behind the 1nonth, exhibits a series of thickenings parallel with one another and transverse to the axis of the body, which may be five, or more, in number, and are termed the '' vjsceral arches." The interspaces between these arches becoming thinner and thinner, are at length perforated by corresponding clefts, whieh place the cavity of the pharynx in free c01nmunication with the exterior. Nothing corresponding with these arches and clefts is known in tbe Invertebrata. A vertebrated animal may be devoid of articulated limbs, and it never possesses more than two pair. These limbs always have an internal skeleton, to whic·h the muscles Inovillg the limbs are attached. Whenever an invert bra ted animal possesses articulated limbs, the skeleton to which the muscles are attached is external, or is connected with an external body skeleton. When an invertebrated animal possesses organs of masti(.'a- |