Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/205

Rh VERTEBRATA 185 tubular cnecum ; p, atriopore ; q, hepatic caecum ; r, intestine ; s, ccelom ; t, area of adhesion between alimentary canal and sheath of notochord ; v, atrial chamber or branchial cavity ; w, post-atrioporal portion of intestine ; x, canals of metapleura exposed by cutting ; B, probe passing through atrio pore into atrial or branchial chamber ; FF 7, probe passed from coeloin, where it expands behind the atriopore, into narrower perienteric coelom of praj- atrioporal region. (3) Portion of (2) enlarged to show atrio-coelomic canals (&quot; brown funnels&quot; of Lankester). 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 ; b, pigment spot (rudimentary eye) on anterior termination of nerve-tube ; c, first pair of nerves in section ; d, fin ray ; &amp;lt;, myotome ; /, notochord; g, space round myotome (? artifact or coelom); /), subchordal canal (? blood-vessel); i, a symmetrical epipleura of pncoral hood. The originally double character of this part of the ventral fin is indicated by the double series of metameric fin rays which support it. It is probable that these &quot;epipleural&quot; folds of Amphioxus correspond to the opercular folds and lateral fins of Craniata. No cartilaginous fin rays are developed in the atrio-pleural (opercular) region 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 Amphioxus 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 gill-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 oil 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 ventral nerves are ininute, and are given off numerously, right and left, from the nerve-tube throughout its length. The dorsal and ventral nerves of a single inyomere 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 upon the left side of the anterior termination of the nerve-cord (see fig. 7, 4). Within the cavity of the nerve-cord at the same point a patch of brown pigment is present (eye-spot). There are 110 repre sentatives of the lateral eyes of Craniata and no otocysts. There is no representative of the Craniates swim-bladder in Amphioxus. 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 &quot; hearts &quot;) 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 &quot;dorsal aorta,&quot; which posteriorly becomes a single vessel. A portal system of veins can be traced in connexion with the hepatic coecum. No system of lymphatic vessels, nor lymphatic &quot;glands,&quot; 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 Craniata. There is no series of nephridia, nor a renal organ formed by the coalescence of nephridia, nor are gonaducts present. The &quot;brown funnels,&quot; a pair of funnel-shaped tubes discovered by Lankester (14), place the ccelomic space of the opercular (epipleural) down- growths of the body-wall in communication with the space which these folds enclose. They appear to be identical with the &quot;collar- pores&quot; of Balanorjlossus, 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 Ascidiaus and as the proboscidean gland and pore of JJalanoylossus. 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 adult 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 (75) describes this condition in &quot;aus*jebildctc&quot; 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 coelom which is carried downwards in the u ft/GHT FIG. 8. Transverse sections of Amph iom s. (Original.) A. Section through re gion of atrio-ccelomic canals, v. B. Section in front of mouth ; the right and left sides are transposed, a, Cavity surrounding tin ray ; a, tin ray ; b, muscular tissue of myotome; c, nerve-cord; d, notochord; e, left aorta; /, thickened ridges of epithelium of prseoral chamber (Rader organ); g, coiled tube lying in a coelomic space on right side of praoral hood, apparently an artery ; h, cuticle of notochord ; i, connective-tissue sheath of notochord ; k, 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 ; o, ditto of metapleur (this should be relatively thicker than it is); q, 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 &quot; tongue &quot; bars alternating with the main bars and devoid of pharyngo- pleural fold and crelom ; it, so-called &quot;dorsal&quot; coelom; x, lymphatic space or canal of metapleur ; y, sub-pharyngeal vascular trunk ; z, blood-vessel (portal vein) on wall of hepatic caecum ; aa, space of atrial or branchial chamber; bb, ventral groove of pharynx (anteriorly this takes the form of a ridge); cc, hyperbranchial groove of pharynx; dd, lumen or space of hepatic ca-cum ; ee, narrow coplomic space surrounding hepatic ca?cum ; //, lining cell-layer of hepatic caecum ; gg, inner face of n pharyngeal bar clothed with hypoblast, the outer face covered with epiblast (represented black) ; hh, a main pharyngeal bar with projecting pharyngeal fold (on which the reference line rests) in section, showing coelomic space beneath the black epiblast; ii, transverse ventral muscle of epipleura ; kk, raphe or plane of fusion of two down-grown epipleura; II, space and nucleated cells on dorsal faceofnoto- ehord ; 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. Thi- gonads are twenty-six pairs in number, corresponding to the 10-30 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 pharynx through its slits, whtn they are ejected by the mouth. In many respects Amphinxus, the only representative of Ccphalo- chorda, bears evidence of being derived from a more highly organized XXIV. 24