English: Amphioxus lauceolatus, Yarrell (Branchiostoma lubricum, Coste). (From Ray Lankester.) (1) Lateral view of adult, to show general form, the myomeres, fin rays and gonads. A, Oral tentacles (28 to 32 in full-grown animals, 20 to 24 in half-grown specimens); B, praeoral hood or praeoral epipleur; C, plicated ventral surface of atrial chamber; D¹, D¹⁷, D²⁶, gonads, twenty-six pairs, coincident with myotomes 10 to 36; E, metapleur or lateral ridge on atrial epipleur; F, atripore, coincident with myotome 36; G¹, G¹⁵, G³⁴, double ventral fin rays, extending from myotomes 37 to 52, but having no numerical relation to them; H, position of anus, between myotomes 51 and 52; I, notochord, projecting beyond myotomes; K⁷, K²⁷, K⁶², myotomes or muscular segments of body-wall, 62 in number; L¹⁰⁰, L²³⁰, L²⁵³, dorsal fin rays, about 250 in number, the hard substance of the ray being absent at 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; N, position of brown funnel of left side (atrio-coelomic canal); O, nerve tube restin on notochord.

(2) Dissection of Amphioxus. By a horizontal incision on each side of the body a large ventral area has been searated 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 opercular folds (wall of atrial or bronchial chamber) by cutting the fluted pharyngo-pleural membrane d, and separated by a vertical cut from the intestinal region. a, Edge of groove formed by adhesion of median dorsal surface of alimentary canal to sheath of notochord; b, median dorsal surface of alimentary canal; c, left dorsal aorta; cc, single dorsal aorta, formed by union of the two anterior vessels; cc' , same vessel resting on intestine; d, cut edge of pharyngo-pleural 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; e', upstanding folds of body-wall (pharyngo-pleural folds) on alternate bars of perforate region of body; f, atrio-coelomic canals or brown funnels (collar-pores of Balanoglossus); g, cavity of a gonad-sac; m, cut musculature of body-wall; n, anus; o, post-atrioporal extension of atrial chamber in form of a tubular caecum; p, atriopore; q, hepatic caecum; r, intestine; s, coelom; 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; E, probe passing through atriopore into atrial or branchial chamber; FF, probe passing from coelom, where it expands behind the atriopore, into narrower perienteric coelom of praeatrioporal region.

(3) Portion of (2) enlarged to show atrio-coelomic canals (“brown funnels” of Lankester). Lettering as in (2).

(4) Section taken transversely through praeoral 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; e, myotome; f, notochord; g, space round myotome (? artifact or coelom); h, subchordal canal (? blood-vessel); i, a symmetrical epipleura of praeoral hood. Illustration from 1911 Encyclopædia Britannica, article Amphioxus.