176 



VERTEBRATA 



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 we 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 peARyNOEAL 

 the family history there may be alternate pro- sacs s slits. 

 gress and degeneration. And so it must be r nu»is«. - 



in the genealogical trees constructed by the b. bj.. U;^ ^ 



naturalist : the fact that a branch is later in M!^l. f^nS 



origin will not imply that it is higher in ela- ?.7^^^^^^^ 

 boration than those below it, and accordingly sac «T 

 we must not expect to draw our tree so as to ~ 



be able to trace all simpler forms to lower 

 ofi'-sets of the tree. 



Divisions The structural features of those animals 



of Verte- -^liict ^ust be admitted to the Vertebrate 



phylum. P^ylu"! in consequence of possessing noto- 

 chord, pharyngeal gill-slits, and dorsal nerve- 

 plate, tubular or unrolled, are such as enable 

 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 



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. 

 Veiy 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. 



1 



SKELETAL VASCOLAR ARCHES. 

 ARCHES. _ / 



SI 



CTPTiptif rplntinns tn nnp nnntbpr nf tbpBP frvnr ^'°- 3-— Three diagrams showing characteristic disposition of gill-slits, skeletal arches, vascular 

 geuBtiL leidtlonS) lO one anotner OI tnese lOUr , arches, and furcal nerves in a primitive ideal Craniate. The following abbreviations require 

 groups we will inquire subsequently for the explanation:— pros., prosencephalon ; (ftoZ., thalamencephalon ; mes., mesencephalon; ?7iei., met- 



present we term these groups "branches." 



encephalon ; aud. 



Charac- 

 ters of 

 Crani- 

 ata. 



They are as follows : 



Phylum Vektebrata. 



Branch a — Craniata (Cuvierian Vertebrata), 



,, 6 — Cephalochorda (Amphioayus). 



,, c — Urochorda {Tunicala). 



„ d — Hemichorda [Balanoglossus). 



The Vertebrata thus limited may be defined as Ccelomate 

 Enterozoa (Metdzoa) 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 dorsaUy by an outgrowth of cells forming the wall 

 of the primitive archenteron ; the notochord may or may 

 not persist in adult life. Pharyngeal gUl-slits, which may 

 or may not persist in adult life, are developed in all Verte- 

 brates. In 9,11, 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 Ckakiata. 



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 extremi^ of the nerve-cord, and rises up 



otocyst or auditory sac ; tni. art.j truncus arteriosus. (Original.) 



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 Cyclosloma. 

 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- 



BF Jn 



Fig. 4. — A. Hypothetical primitive Fish, with continuous lateral fins S, S (paired 

 right and left), confluent with median azygos fin (An), the post-anal part of 

 which is marked S', whilst its dorsal part is marked D. B. Actual Fish, 

 showing relation of isolated lateral and median fins to original hypothetical 

 fins of the upper figure. BrP, left pectoral fln (paired) ; BF, left pelvic fin 

 (paired) ; AF, anal (post-anal) azygos fin ; SF, caudal azygos fin ; EF and 

 FF, anteriorandposteriorazygosdorsalfins; An, anus. (FromWiedersheim.) 



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 



