180 



VERTEBRATA 



tubnlar caecum ; p, atriopore ; g, hepatic cfficum ; r, intestine ; s, ccelom ; t, 

 area of adhesion between alimentary canal and sheath of njotochoni ; v, atrial 

 chamber or branchial cavity; w, post-atrioporal portion of intestine; a:, 

 canals of metapleura exposed by cutting ;. E, probe passing through atrio- 

 pore into atrial or branchial chamber ; FF, probe passed from ccelom, where 

 it expands behind the atriopore, into narrower perienteric ccelom of prse- 

 atrioporal region. 



(S) Portion of (2) enlarged to show atrio-ccBlomic canals (" brown funnels " 

 of Jjankester). Lettering as in (2). 



(4) Section taken transversely through prseoral region near termination of 

 nerve-tube, a, Olfactory ciliated pit on animal's left side, its wall confluent 

 with substance of nerve-tube ; 6, pigment spot (rudimentary eye) on anterior 

 termination of nerve-tube ; c, first pair of nerves in section ; d, fin ray ; e, 

 myotome ; /, notochord ; j7, space round myotome (? artifact or ccelom) ; A, 

 snbchoi'dal canal (? blood-vessel); i, a symmetrical epipleura of prgeoral hood. 



The originally double character of this part of the ventral fln is 

 indicated by the double series of metameric fin rays which support 

 it. It is probable that these "epipleural" folds of Amphioxus 

 correspond to the opercular folds and lateral fins of Craniata. No 

 cartilaginous fin rays are developed in the atrio-pleural (opercular) 

 legion of the epipleura ; but a longitudinal unsegmented bar of 

 cartilaginous consistency strengthens its side and bounds a lymph- 

 holding canal {x in fig. 8). 



The gill-slits in AmpMoxiis are very numerous (one hundred or 

 more), and have no numerical relation to the metameres of the 

 muscular body -wall, though the first few which appear in the 

 embryo correspond at the time to successive myomeres, — a relation 

 which they subsequently lose. The sides of the giU-slits are sup- 

 ported by chitinous (?) bars, and each slit is divided into two equal 

 portions by a longitudinal tongue or bar, which grows out from 

 the dorsal margin of the slit soon after its first formation. The 

 number of gill-slits increases continually throughout the life of 

 Amphioxus by the formation of new ones at the posterior border of 

 the pharynx, whilst the myomeres do not increase in number after 

 early embryonic life. 



The nerves given off from the dorsal nerve-cord of Amphioxus are 

 of two kinds, — dorsal and ventral. The dorsal nerves correspond 

 in number and position to the myomeres, right and left, except in 

 the most anterior region of the body, where two larger pairs of 

 dorsal nerves are given oflf from near the extremity of the nerve- 

 cord, and pass forward, supplying the region which lies in front of 

 the termination' of the musculature. The central nerves are 

 minute, and are given off numerously, right and left, from the 

 nerve-tube throughout its length. The dorsal and ventral nerves 

 of a single myomere appear to correspond, respectively, to the 

 dorsal and ventral roots of a spinal nerve of a Craniate. 



There is a single olfactory pit in Amphioxus, which rests ujion 

 the left side of the anterior termination of the nerve-cord (see fig. 

 7, i). Within the cavity of the nerve-cord at the same point a 

 patch of brown pigment is present (eye-spot). There are no repre- 

 sentatives of the lateral eyes of Oraniata and no otocysts. 



There is no representative of the Craniates' swim-bladder in 

 Amphioxiis. A single wide diverticulum of the alimentary canal 

 represents the liver of Craniata ; the pancreas is unrepresented. 



The vascular system is singularly incomplete : large trunks 

 exist, but few branches and no heart, whilst the blood itself is 

 colourless, and communicates (as in Craniata by the lymphatic 

 "hearts ") with the ccelomic fluid at various points. A contractile 

 ventral trunk runs along the lower face of the slit pharynx, and 

 sends vessels right and left up the successive bars ; these vessels 

 unite above, as in Craniata, to form a double " dorsal aorta," which 

 posteriorly becomes a single vessel. A portal system of veins can 

 be traced in connexion with the hepatic caecum. 



No system of lymphatic vessels, nor lymphatic "glands," nor a 

 spleen exist ; but the ccelom, and certain other spaces in the 

 connective tissue, contain coagulable lymph, and correspond to the 

 lymph spaces of Oraniata. 



There is no series of nephridia, nor a renal organ formed by the 

 coalescence of nephridia, nor are gonaducts present. The " brown 

 funnels," a pair of funnel-shaped tubes discovered by Laiikester 

 {14), place the ccelomic space of the opercular (epipleural) dovm- 

 growths of the body-wall in communication with the space which 

 these folds enclose. They appear to be identical with the "collar- 

 pores" oi Balanoglossus, and it is doubtful whether they represent 

 nephridia. 



In the larval Amphioxus there is developed from the left anterior 

 ccelomic pouch a glandular tube and a sense-organ, which are re- 

 presented in the adult by the structures marked/ in fig. 8, B.' This 

 tube is probably the same thing as the subneural gland of Ascidians 

 and as the proboscidean gland and pore of Balanoglossus. Quite 

 distinct from the foregoing is a nephridial tube lying on the left 

 side behind the mouth of the larval Amphioxus. All are probably 

 of the nature of nephridia. In the aSmM Amphioxus the nephridial 

 tube is in an atrophied condition, though large and active in a late 

 larval stage, when the olfactory pit opens into the neural canal. 

 Hatschek (jy) describes this condition in " ausgebildete" but not 

 in adult examples. 



The gonads are distinct ovaries and testes ; they are developed 

 in distinct male and female individuals in corresponding positions, 

 viz., in that part of the ccelom which Is carried downwards in the 



a/CHT 



Fig. 8. — ^Transverse sections of Aw^i^aeia, (Original.) Ju Section through re- 

 gion of atrio-ccelomic canals, t>. B. Section in front of mouth ; the right and 

 left sides are transposed, a. Cavity surrounding fin ray ; a', fin lay ; 6, 

 muscular tissue of myotome ; c, nerve-cord ; d, notochord ; e, left aorta ; f, 

 thickened ridges of epithelium of praioral chamber (Rader organ); g, coiled 

 tube lying in a ccelomic space on righi side of prseoral hood, apparently an 

 artery; A, cuticle' of notochord ; i, connective-tissue sheath of notochord; fc, 

 median ridge of skeletal canal of nerve-cord ; I, skeletal canal protecting 

 nerve-cord ; m, inter-segmental skeletal septum of myotome ; n, subcutaneous 

 skeletal connective tissue ; 0, ditto of metapleur (this should be relatively 

 thicker than it is) ; g, subcutaneous connective tissue of ventral surface of 

 atrial wall (not a canal, as supposed by Stieda and others) ; r, epiblastic epi- 

 thelium ; s, gonad-sac containing ova ; t, pharyngeal bar in section, one of 

 the "tongue" bars alternating with the main bars and devoid of pharyngo- 

 pleural fold and ccelom ; v, atrio-ccelomic funnel; w, so-called "dorsal "ccelom; 

 X, lymphatic space or canal of metapleur ; ^, sub-pharyngeal vascular tnmk ; z, 

 blood-vessel (portal vein) on wall of hepatic csecum ; aa^ space of atrial or 

 branchial chamber ; &6, ventral groove of pharynx (anteriorly this takes the 

 form of a ridge) ; cc^ hyperbranchial gro6ve of pharynx ; dd, lumen or space of 

 hepatic csecum; ee, naiTow ccelomic space surrounding hepatic csecum; ff, 

 lining cell-layer of hepatic csecum ; gg, inner face of a pharyngeal bar clothed 

 with hypoblast, the outer 'ace covered with epiblast (represented black); AA, a 

 main pharyngeal bar with projecting pharyngeal fold (on which the reference 

 line rests) in section, showing ccelomic space beneath the black epiblast ; ii, 

 transverse ventral muscle of epipleura ; fcfc, raphe or plane of Aision of two 

 down-grown epipleura ; II, space and nucleated cells on dorsal face of noto- 

 chord ; mm, similar space and cells on its ventral face. 



descending right and left outgrowth (epipleura, opercula) of the 

 body-wall, which encloses the atrial or branchial chamber. The 

 gonads are twenty-six pairs in number, corresponding to the 10-36 

 myomeres. They are devoid of ducts, and discharge their pro- 

 ducts by dehiscence into the atrial chamber, whence they pass to 

 the exterior, either by the atriopore or by entering the phaiynx 

 through its slits, when they are ejected by the mouth. 



In many respects Amphioxus, the only representative of Cephalo- 

 chorda, bears evidence of being derived from a more highly organized 



