through these clefts into the large surrounding branchial cavity or
atrium, and then pours out behind through a hole in it, the
respiratory pore (porus branchialis, Figure 2.210 c). Below, on the
ventral side of the gill-crate, there is in the middle line a ciliated
groove with a glandular wall (the hypobranchial groove), which is also
found in the Ascidia and the larvae of the Cyclostoma. It is
interesting because the thyroid gland in the larynx of the higher
vertebrates (underneath the "Adam's apple") has been developed from
it.

(FIGURE 2.212. Transverse section of an Amphioxus-larva, with five
gill-clefts, through the middle of the body.

FIGURE 2.213. Diagram of the preceding. (From Hatschek.) A epidermis,
B medullary tube, C chorda, C1 inner chorda-sheath, D visceral
epithelium, E sub-intestinal vein. 1 cutis, 2 muscle-plate (myotome),
3 skeletal plate (sclerotome), 4 coeloseptum (partition between dorsal
and ventral coeloma), 5 skin-fibre layer, 6 gut-fibre layer, I myocoel
(dorsal body-cavity), II splanchnocoel (ventral body-cavity).)

Behind the respiratory part of the gut we have the digestive section,
the trunk or liver (hepatic) gut. The small particles that the
Amphioxus takes in with the water--infusoria, diatoms, particles of
decomposed plants and animals, etc.--pass from the gill-crate into the
digestive part of the canal, and are used up as food. From a somewhat
enlarged portion, that corresponds to the stomach (Figure 2.210 e), a
long, pouch-like blind sac proceeds straight forward (f); it lies
underneath on the left side of the gill-crate, and ends blindly about
the middle of it. This is the liver of the Amphioxus, the simplest
kind of liver that we meet in any vertebrate. In man also the liver