English: Anterior region of two pelagic larvae of Amphioxus lanceolatus obtained by the tow-net in 8-10 fathoms, showing the asymmetry of the large lateral sinistral mouth with its ciliated margin cm and the dextral series of simple primary gill-slits (1ps-14ps). The larvae swim normally like the adult or suspend themselves by their flagella (not shown in the figures) vertically in mid-water. There is nothing in their mode of life which will afford an explanation of the asymmetry which is a developmental phenomenon.
Lettering of upper figure.—anp, Anterior neural pore; bc, rudiment of buccal skeleton; c, cilia; cb, ciliated band; cc, ciliated groove; cm, cilia at margin of mouth; gl, external opening of club-shaped gland; Hn, Hatschek's nephridium; lm, left metapleur; n, notochord; pp, praeoral pit; ps, primary gill-slits, 1, 5, and 13; rm, right metapleur showing through.
Lettering of lower figure.—a, Atrium; al, alimentary canal; bv, blood-vessel; cv, cerebral vesicle; df, dorsal section of myocoel (=fin spaces); e, “eyespot”; end, endostyle; gl, club-shaped gland; lm, edge of left metapleur; m, lower edge of mouth; n, notochord; nt, pigmented nerve tube; ps, primary gill-slits, 1, 9, and 14; rc, renal cells on atrial floor; rm, edge of right metapleur; so, sense organ opening into praeoral pit; ss, thickenings, the rudiments of the row of secondary gill-slits.
Illustration from 1911 Encyclopædia Britannica, article Amphioxus.