ph spaces, the metapleural
canals, between which a small invagination (i.e., a pushing-in), at., of
the outer epidermis occurs; n.c. is the notochord, and s.c. the spinal
cord. The gill slits, by which P. communicates with the exterior, are
not shown. Next Figure 11 shows the invagination (at.) pushing its
way in, and cut off from the exterior by a meeting of the body wall
below. Note that at. is a portion of the animal's exterior thus
embraced by its body, and that its lining is therefore of the same
material as the external integument. In Figure 12, at. is developing
upward, so that the true body hangs into it. Now imagine the gill slits
perforated, as shown by the double-headed arrow in Figure 12. Figure
3, on Sheet 20, is a less diagrammatic representation of a
cross-section of the pharyngeal region (vide Figure 1, Sheet 19). The
student should compare Figure 3, Sheet 20, and Figure 12, Sheet 21.
The atrium and metapleural canals are easily recognised in both. In
Figure 3 the coelom is much cut up by the gill slits, and we have
remaining of it (a) the dorsal coelomic canals (d.c.c.) and (b) the
branchial canals (br.c.) in the bars between the slits. The atrial cavity
remains open to the exterior at one point, the atrial pore (at.p.).

Section 7. The method of examining cross-sections is an extremely
convenient one in the study of such a type as Amphioxus. The
student should very carefully go over and copy the six sections on
Sheet 20, comparing Figure 1 as he goes. He should do this before
reading what follows. One little matter must be borne in mind. These
figures are merely intended to convey the great structural ideas, and
they are considerably simplified; they must not be regarded as a
substitute for the examination of microscopic sections. [He will notice
a number of rounded masses from the body wall. The] -For instance,
the body-wall- muscles of Amphioxus are arranged in bundles bent
sharply in an arrow shape, the point forward. -A number of these
bundles are cut in any one section, and so the even shading of our
diagrams, if they professed to be anything more than diagrams,
should be broken up into masses.- These -bundles, we may mention-,
are called myomeres, and they are indicated in Figure 1 by lines
pointing acutely forward. [Several are consequently cut in any
transverse section (Sheet 20), and these are the rounded masses he
sees.] Similar myomeres, similarly situated, are found in fish, behind
the h