VERTEBRATA 



179 



strable that two originally lateral cords have coalesced tentrally to 

 form the Annelid's ventral nerve-chain. 



The comparison of the Vertebrates' nervous system with that of 

 the Nemertines had already been made by the present writer, as 

 cited by Hubrecht (//) in connexion with the latter's discovery of 

 a complete sub-epidermal nerve-tunic in those worms. Hubrecht 

 has more recently on two occasions (12 and 13) developed an in- 

 teresting and important comparison of Nemertine and Vertebrate 

 structure. He has in the first place suggested that the notochord 

 of Vertebrata is nothing more than a modified survival of the pro- 

 boscidean sheath of the Nemertines, whilst the oral invagination of 

 the epidermis, in connexion with the hypophysis eerebri of the 

 Vertebrate, may be a last remnant of the proboscis itself. More 

 conclusively he has drawn attention to the median dorsal nerve of 

 Nemertines as corresponding ^o the Vertebrate cerebro-spinal nerve- 

 cord, whilst the great lateral nerve-cords of Nemertines, and the 

 lateral ganglia in which they expand anteriorly, are compared to 

 the lateral ganglia of the cephalic region of Craniate Vertebrata 

 and the nerve of the lateral line (see fig. 6). The comparison is 

 strengthened by the existence of a inetameric series of transverse 

 nerves in the Nemertine, which correspond in respect of their meta- 

 merism and their connexion with a dorsal median trunk, with the 

 spinal nerves of Craniata. Hubrecht is careful to insist that he 

 does not regard the Nemertines as representing the direct ancestry 

 of Vertebrata ; but he points out that from the primitive condition 

 of an elongate animal, with a plexiform nerve-tunic, it is readily 

 conceivable that a form was developed in which the nervous tissue 

 was concentrated in three cords, a median dorsal and two lateral, 

 and from such a form we can derive the Craniates' condition by 

 excessive development of the median tract and relatively small 

 development of the lateral cords, whilst the Nemertines' condition 

 would be attained by the converse process. The tubular condition 

 of the cerebro-spinal nerve-cord of Vertebrata, it may here be re- 

 marked, is now very generally regarded as being in its origin a 

 purely developmental feature. It was primitively separated from 

 the epidermis by delamination and in -sinking, and the mode of 

 formation by invagination of a canal has been substituted in accord- 

 ance with a general embryological law of growth, which is that 

 bulky structures originating beneath a surface from the cells form- 

 ing that surface take up their position in embryonic growth by in- 

 vagination of the parent surface. The tubular form, having thus 

 started, seems to have been utilized during one phase of Vertebrate 

 evolution for the respiration of the nervous tissue, by the introduc- 

 tion through an anterior unclosed pore of a current of water, which 

 escaped by the neuranal canal (as in larval Amphioxus). 



There is a wide gap between any form presenting an approach to 

 a Nemertine Worm and the most simple Craniate Vertebrate which 

 can be imagined still provided with the organization characteristic 

 of all Craniata. To pass from such a Worm-like animal to a Craniate, 

 we have to account for and introduce, amongst other new develop- 

 ments, (1) a greatly increased metamerism, showing itself in the 

 segmentation of the muscles of the body-wall and in the repetition 

 of the nephridia ; (2) the characteristic sense organs; (3) the lateral 

 and median longitudinal folds or continuous fins ; (4) the carti- 

 laginous rods and bars of the skeleton ; (5) the gill-slits, even if 

 we admit the notochord to be represented by the proboscidean 

 sheath. 



It remains to inquire whether the structure of the other Verte- 

 brata throws light on this long hypothetical passage from the 

 simple Worm phase to the elaborate Craniate, or suggests any other 

 ancestry. 



THE CEPHALOCHORDA. 



Char- CephalocJiorda are Vertebrata in which there is no anterior 



acters of dilatation of the nerve-tube to form a brain (see fig. 6) aud no 

 Cephalo- specialized skeletal brain-case. The notochord extends from one 

 chorda, extremity of the elongate body to the other as a tapering nncon- 

 stricted rod, passing anteriorly some distance in front of the 

 nerve-cord. The longitudinal muscles of the body -wall are divided 

 by transverse fibrous septa into a series of segments (sixty-two in 

 Amphioxus laneeolatus), the more anterior of which are in front 

 of the mouth and not in any way fused to form a head or cranial 

 structure. Dense connective tissue (differing but little from car- 

 tilage) forms an unsegmented sheath to the notochord and an 

 unbroken neural canal above it, in which the nerve-cord lies. 

 The same tissue forms a series of metamerically repeated fin rays, 

 which support the base of a median fin extending along the entire 

 dorsal surface. The fin is continued ventrally from the caudal ex- 

 tremity as far forward as the anus, but without fin rays. Two 

 lateral up-growths of the body-wall (the epipleura) extend one on 

 either side from the head as far back as the anus. Each of these 

 is divided into three regions, (1) an anterior, which forms the 

 praeoral hood ; (2) a median, which forms the wall of the great 

 branchial chamber, the two folds meeting one another and coales- 

 cing in the ventral mid-line, excepting where they leave a posterior 

 median aperture, the atriopore ; and (3) the post-atrioporal pneanal 

 ventral fin (extending between atriopore and anus), which is formed 

 by the complete coalescence of the two folds behind the atriopore. 



FlO. 7. Awtphiaxus Janaolatus, Yarrell (Branchiostoma lubrinm, Coste\ 

 (Original drawings.) (1) lateral view of adult, to show general form, the 

 myomeres, fin rays, and gonads. A, oral tentacles (28 to 32 in fall -grown 

 animals, 20 to 24 in half-grown specimens) ; B, prseoral hood or pneoral epi- 

 pleur ; C, plicated ventral surface of atrial chamber ; Di, D", D 5 *, gonads, 

 twenty-six pairs, coincident with myotomes 10 to 36 ; B, metapleor or lateral 

 ridge on atrial epiplenr ; P, atriopore, coincident with myotome 36 ; Gi, GH, 

 G**, double ventral fin rays, extending from myotomes 37 to 52, but hav- 

 ing no numerical relation to them ; H, position of anas, between myotomes 

 51 and 52 ; I, notochord, projecting beyond myotomes ; K^, K", K", myo- 

 tomes or muscular segments of body-wall, 62 in number ; LW*, L 2 **, L 253 , dor- 

 sal fin rays, about 250 in number, the hard substance of the ray being absent 

 st the extreme ends of the body (these have no constant numerical relation 

 to the myomeres); M, notochord as seen through the transparent myotomes, 

 the thin double-lined spaces being the connective-tissue septa and the 

 broader spaces the muscular tissue of the myotomes ; If, position of brown 

 funnel of left side (atrio-coclomic canal) ; O, nerve-tube resting on notochord. 

 (2) Dissection of Amphicana. By a horizontal incision on each side of the 

 body a large ventral area has been separated and turned over, as it were on 

 a hinge, to the animal's left side. The perforated pharyngeal region has then 

 been detached from the adherent epipleura or opercnlar folds (wall of atrial 

 or branchial chamber) by cutting the fluted pharyngo-pleural membrane rf, 

 and separated by a vertical cnt from the intestinal region, a. Edge of groove 

 formed by adhesion of median dorsal surface of alimentary canal to sheath 

 of notochord ; o, median dorsal surface of alimentary canal ; c, left dorsal 

 aorta ; oc, single dorsal aorta, formed by union of the two anterior vessels ; of, 

 same vessel resting on intestine ; d, cnt edge of pharyngo-plenral folds of 

 atrial tunic, really the original outer body-wall before the downgrowth of 

 epipleura ; d% atrial tunic (original body-wall) at non-perforate region, cut 

 and turned back so as to expose peri-enteric coelom and intestine r ; t, up- 

 standing folds of body-wall (pharyngo-pleural folds) on alternate bars of pep 

 forate region of body ; /, atrio-coelomic canals or brown funnels (collar-pores 

 of l!alanog!ossus) ; g, cavity of a gonad-sac ; m, cut musculature of body- 

 wall ; n, anus ; o, post-atrioporal extension of atrial chamber in form of 



