Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/201

Rh VERTEBRA T A 181 simplest form on the lowest step ; nor can we be satisfied with a tree-like arrangement, in which the forms at the ends of the branches are always more elaborate than those nearer the trunk. Our genealogical tree will more strictly conform to that of a parvenu human family, if Ave take worldly prosperity in the latter case as corresponding to elaboration of structure in the former. The strict family genealogist will include in the successive ramifications of the tree the five sons of the founder of the family, one of whom remained an agricultural labourer, whilst two be came brewers and two emigrated. The cousins in the next generation will be set forth in place, the sons of one brewer becoming paupers, whilst those of the other advance to the position of Government employes, and one to the peerage. Thus in successive branchings of _. i i i i t i rHARYNGKAL the family history there may be alternate pro- SACS & SLITS. gress and degeneration. And so it must be R N ai sac. in tlie genealogical trees constructed by the R, Eye. naturalist : the fact that a branch is later in origin will not imply that it is higher in ela boration than those below it, and accordingly we must not expect to draw our tree so as to be able to trace all simpler forms to lower 3t off-sets of the tree. Divisions The structural features of those animals of Verte- w hicll must be phylum in consequence ot possessing laterally so as to enclose and protect the brain (hence Craniata]. Cartilage is developed in other parts of the body as a skeletal sub stance, though it may be subsequently replaced in the cranium, as elsewhere, by bone. The longitudinal muscles of the body-wall are divided by transverse fibrous septa into a series of segments, vary ing in the adult from ten to one hundred or more in number. Cartilaginous neural arches, corresponding in number and position to the fibrous septa, and resting on the notochord, are developed so as to protect the nerve-cord. Cartilaginous bars also pass out wards, with a direction at first horizontal and then ventral, from the sides of the notochord into the intermuscular fibrous septa. Very generally, but not always, a tubular cartilaginous sheath forms round the notochord ; this sheath with rare exceptions be comes segmented to form a series of vertebral bodies, which lie in the planes of the fibrous intersegmental septa, and, increasing in thickness by encroaching upon the substance of the notochord, finally obliterate it almost entirely. SKELETAL, ARCHES. phylum Spinal, admitted to the Vertebrate of possessing noto chord, pharyngeal gill-slits, and dorsal nerve- plate, tubular or unrolled, are such as enable 8th us very readily to group them in four great divisions, which appear to be equally distinct from one another. As to what may be the o-priptir relation** tn nnp nnorripr- nf thpqp fniir FlG ?-- Three diagrams showing characteristic disposition of gill-slits, skeletal arches, vascular arches, and furcal nerves in a primitive ideal Craniate. The following abbreviations require groups WO will inquire Subsequently; for the explanation: pros., prosencephalon ; toaZ., thalamencephalon; mes., mesencephalon ; met., met- ,1 &amp;lt;i i i. encepualou; aud., otocyst or auditory sac ; tru. art., trimcus arteriosus. (Original.) these groups &quot; branches. &quot; The pharyngeal slits follow closely upon the mouth, and in ex isting Craniata never number more than eight pairs (see fig. 3). They are separated from one another and their apertures strengthened by a series of cartilaginous hoops, the first of which, that between the mouth and the first gill-slit, forms the primitive upper and lower jaw in all but the small and degenerate group Cyclostoma. The gill-slits when functional are generally protected by an opercular fold of the body-wall, which overhangs them and corresponds to the epipleural fold of Amphioxus, the collar of Balanoglossus, and doubtfully to the wall of the atrial chamber of Urochorda. The extension of this fold along the sides of the middle third of the body (between the pharyngeal region and the anus) acquired in ancestral Craniata the function of a continuous right and left lateral fin (see fig. 4). At the same time a continuous median fin, corre- Charac- ters of Crani ata. present we term They are as follows : Phylum VERTEBRATA. Branch a Craniata (Cuvierian Vertebrata}. ,, b Cephalockorda (Amphioxus), ,, c Urochorda ( Tunicata ) . ,, d -Hemichorda (Balanoijlossns). The Vertebrata thus limited may be defined as Coelomate Enterozoa (Metazoa) with well-developed coelom. In all, with the exception of the more degenerate members of Urochorda, an elastic skeletal rod the notochord is de veloped dorsally by an outgrowth of cells forming the wall of the primitive archenteron ; the notochord may or may not persist in adult life. Pharyngeal gill-slits, which may or may not persist in adult life, are developed in all Verte brates. In all, except in certain Urochorda, the chief nerve- centre has the form of a dorsal, median, elongate tract, derived from the epiblast, which becomes sunk below the surface and invaginated so as to form a tube. In all there is a tendency to metameric repetition of parts, which may find its expression in a strongly-marked segmentation of the musculature of the body-wall and its skeleton, or may be recognizable only in a limited degree, as exhibited by the successive gill-slits or successive gonads. We shall now examine the distinctive features of each of these large groups, and form an estimate of their rela tions to one another, and of their probable ancestry, this being the task to which we must limit ourselves in the brief space here afforded. THE CRAXIATA. The Craniata are Vertebrata in which the tubular cerebro-spinal nerve-mass is swollen anteriorly to form a brain, consisting primarily of three successive vesicles, in connexion with the anterior of which the special nerves of the olfactory organs and of the eyes originate. The notochord, whilst extending posteriorly to the extremity of the body, does not reach quite so far forward anteriorly as the termina tion of the nerve-tube. A cartilaginous cranium or brain-case de velops round the anterior extremity of the nerve-cord, and rises up Bl&quot; An Fio. 4. A. Hypothetical primitive Fish, with continuous lateral fins S, S (paired right and left), confluent with modiaii azygos fin (An), the post-anal part of which is marked S, whilst its dorsal part is marked I). B. Actual Fish, showing relation of isolated lateral and median fins to original hypothetical /ins of the upper figure. BrF, left pectoral liu (paired); BF, left pelvic tin (paired); AF, anal (post-anal) azygos tin; SF, caudal azygos tin; KF and FF, auteriorand posterior azygos dorsal tins ; An, anus. (From Wiedersheim.) spending to the dorsal, caudal, and anal fins of existing Fishes, was developed. In both lateral and median fins a cartilaginous skeleton was developed, consisting of a basal longitudinal bar, supporting a number of rods like the teeth of a comb. The primitive form of fin skeleton is retained in the median fins of some sharks ; the primitive lateral fin has in all cases either