Page:EB1911 - Volume 01.djvu/939

 {| External Form.—The following description, unless otherwise stated, refers to A. lanceolatus. Amphioxus is a small fish-like creature attaining a maximum length of about 3 in., semi-transparent in appearance, showing iridescent play of colour. The body is narrow, laterally compressed and pointed at both ends. The main musculature can be seen through the thin skin to be divided into about sixty pairs of muscle-segments (myotomes) by means of comma-shaped dissepiments, the myocommas, which stretch between the skin and the central skeletal axis of the body. These myotomes enable it to swim rapidly with characteristic serpentine undulations of the body, the movements being effected by the alternate contraction and relaxation of the longitudinal muscles on both sides. Apparently correlated with this peculiar locomotion is the anatomical fact of the alteration of the myotomes on the two sides. Symmetrical at their first appearance in the embryo, the somites (from which the myotomes are derived) early undergo a certain distortion, the effect of which is to carry the somites of the left side forwards through the length of one half-segment. For example, the twenty-seventh myotome of the left side is placed opposite to the twenty-sixth myocomma of the right side. The back of the body is occupied by a crest, called the dorsal fin, consisting of a hollow ridge, the cavity of which is divided into about 250 compartments or fin chambers into each of which, with the exception of those near the anterior and posterior end of the body, projects a stout pillar composed of characteristic laminar tissue, the fin ray. The dorsal crest is continued round both extremities, becoming expanded to form the rostral fin in front and the caudal fin behind. Even in external view, careful inspection will show that the body is divisible into four regions, namely, cephalic, atrial, abdominal and caudal. The cephalic region includes the rostrum or praeoral
 * 1911 Britannica - Transverse sections of Amphioxus.png
 * .—Transverse sections of Amphioxus. (From Lankester.) A. Section through region of atrio-coelomic canals, v. B. Section in front of mouth; the right and left sides are transposed. a, Cavity surrounding fin ray; a' , fin ray; b, muscular tissue of myotome; c, nerve-cord; d, notochord; c, left aorta; f, thickened ridges of epithelium of praeoral chamber (Rader organ); g, coiled tube lying in a coelomic space on right side of praeoral hood, apparently an artery; h, cuticle of notochord; i, connective-tissue sheath of notochord; k, median ridge of skeletal canal of nerve-cord; l, 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, sub-cutaneous connective tissue of ventral surface of atrial wall (not a canal, as supposed by Stieda and others); r, epiblastic epithelium; 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 coelom; v, atrio-coelomic funnel; w, socalled “dorsal” 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 caecum; ee, narrow coelomic space surrounding hepatic caecum; ff, lining cell-layer of hepatic caecum; gg, inner face of a 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; ll, space and nucleated cells on dorsal face of notochord; mm, similar space and cells on its ventral face.
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