end we have a sort of lattice work of fine gill-clefts, supported on a
number of stiff branchial rods; these are connected in pairs by
transverse rods.

(FIGURES 2.222 TO 2.224. Transverse sections of young Amphioxus-larvae
(diagrammatic, from Ralph.) (Cf. also Figure 2.216.) In Figure 2.222
there is free communication from without with the gut-cavity (D)
through the gill-clefts (K). In Figure 2.223 the lateral folds of the
body-wall, or the gill-covers, which grow downwards, are formed. In
Figure 2.224 these lateral folds have united underneath and joined
their edges in the middle line of the ventral side (R seam). The
respiratory water now passes from the gut-cavity (D) into the
mantle-cavity (A). The letters have the same meaning throughout: N
medullary tube, Ch chorda, M lateral muscles, Lh body-cavity, G part
of the body-cavity in which the sexual organs are subsequently formed.
D gut-cavity, clothed with the gut-gland layer (a). A mantle-cavity, K
gill-clefts, b = E epidermis, E1 the same as visceral epithelium of
the mantle-cavity, E2 as parietal epithelium of the mantle-cavity.)

At an early stage of embryonic development the structure of the
Amphioxus-larva is substantially the same as the ideal picture we have
previously formed of the "Primitive Vertebrate" (Figures 1.98 to
1.102). But the body afterwards undergoes various modifications,
especially in the fore-part. These modifications do not concern us, as
they depend on special adaptations, and do not affect the hereditary
vertebrate type. When the free-swimming Amphioxus-larva is three
months old, it abandons its pelagic habits and changes into the young
animal that lives in the sand. In spite of its smallness (one-eighth
of an inch), it has substantially the same structure as the adult. As
regards the remaining organs of the Amphioxus, we need only mention