idae_) there is a long
capillary vermiform extension of the stomochord in front. The nuchal
skeleton is a non-cellular laminated thickening of basement-membrane
underlying that portion of the stomochord which lies between the
above-mentioned pouches and the orifice into the throat. At the point where
the stomochord opens into the buccal cavity the nuchal skeleton bifurcates,
and the two cornua thus produced pass obliquely backwards and downwards
embedded in the wall of the throat, often giving rise to projecting ridges
that bound a dorsal groove of the collar-gut which is in continuity with
the wall of the stomochord (fig. 3).

_Nervous System_.--At the base of the epidermis (which is in general
ciliated) there is over the entire surface of the body a layer of
nerve-fibres, occurring immediately outside the basement-membrane which
separates the epidermis from the subjacent musculature. The nervous system
is thus essentially epidermal in position and diffuse in distribution; but
an interesting concentration of nerve-cells and fibres has taken place in
the collar-region, where a medullary tube, closed in from the outside,
opens in front and behind by anterior and posterior neuropores. This is the
collar nerve-tube. Sometimes the central canal is wide and uninterrupted
between the two neuropores; in other cases it becomes broken up into a
large number of small closed medullary cavities, and in others again it is
obsolete. In one family, the _Ptychoderidae_, the medullary tube of the
collar is connected at intermediate points with the epidermis by means of a
variable number of unpaired outgrowths from its dorsal wall, generally
containing an axial lumen derived from and in continuity with the central
canal. These hollow roots terminate blindly in the dorsal epidermis of the
collar, and place the nervous layer of the latter in direct connexion with
the fibres of the nerve-tube. The exact significance of these roots is a
matter for speculation, but it seems possible that they are epiphysial
structures remotely comparable with the epiphysial (pineal) complex of the
craniate vertebrates. In accordance with this view there would be also some
probability in favour of regarding the collar nerve-tube of the
Enteropneusta as the equivalent of the cerebral vesicle only of _Amphioxus_
and the Ascidian tadpole, and also of the primary fore-brain of
vertebrates.

Special thickenings of the diffuse nervous layer of the epidermis occur in