English: 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. Illustration from 1911 Encyclopædia Britannica, article
Amphioxus.