lus, from which it issues
by efferent vessels which effect a junction with the ventral
(sub-intestinal) vessel in the trunk. The vascular system does not readily
lend itself to morphological comparison between such widely different
animals as _Balanoglossus_ and _Amphioxus_, and the reader is therefore
referred to the memoirs cited at the end of this article for further
details.

[Illustration: FIG. 3.--Structure of anterior end.

a, Arrow from proboscis-cavity (pc) passing to left of pericardium (per)
and out through proboscis pore-canal.
b^1, arrow from central canal of neurochord (cnc) passed out through
anterior neuropore.
b^2, ditto; through posterior neuropore.
c, arrow intended to pass from 1st gill-pouch through collar pore-canal
into collar-coelom (cc).
cts, posterior limit of collar.
dv, dorsal vessel passing into central sinus (bs).
ev, efferent vessel passing into ventral vessel (vv).
epr, epiphysial tubes.
st, stomochord.
vs, ventral septum of proboscis.
sk, body of nuchal skeleton.
m, mouth.
th, throat.
tb, tongue-bars.
tc, trunk coelom. ]

_Reproductive System_.--The sexes are separate, and when mature are
sometimes distinguished by small differences of colour in the genital
region. Both male and female gonads consist of more or less lobulated
hollow sacs connected with the epidermis by short ducts. In their
disposition they are either uniserial, biserial or multiserial. They occur
in the branchial region, and also extend to a variable distance behind it.
In exceptional cases they are either confined to the branchial region or
excluded from it. When they are arranged in uniserial or biserial rows the
genital ducts open into or near the branchial grooves in the region of the
pharynx and in a corresponding position in the post-branchial region. An
important feature is the occurrence in some species (_Ptychoderidae_) of
paired longitudinal pleural or lateral folds of the body which are mobile,
and can be approximated at their free edges so as to close in the dorsal
surface, embracing both the median dorsal nerve-tract and the branchial
grooves with the gill-pores, so as to form a temporary peri-branchial and
medullary tube, open behind where the folds cease. On the other hand, they
can be spread out horizontally so as to expose their own upper side as well
as the dorsal surface of the body (fig. 1). These folds are called the
genital pleurae because they contain the bulk of the gonads. Correlated