lly (as in the rabbit) by at first cartilaginous vertebrae whose
bodies are derived from its sheath. Further, while in all true vertebrata
the notochord of the developing young reaches anteriorly at most to
the mid-brain, and is there at its termination enclosed by the middle
portion of the skull, in Amphioxus it reaches far in front of the anterior
extremity of the nervous system, to the end of the animal's body.*
On this account the following classification is sometimes made of
those animals which have a notochord:--

-Chordata_ (= Vertebrata, as used by Lankester).

1. Having the notochord reaching in front of the brain.
Cephalochorda = Amphioxus.

2. Having the notochord reaching anteriorly to the mid-brain, a
brain of three primary vesicles and a skull.
Craniata = all "true vertebrata": fishes, amphibia, reptiles,
birds, and mammals (Vertebrata of Balfour).

3. Having the notochord confined to the tail.
Urochorda = the ascidians, or sea-squirts, certain forms of life
only recently recognised as relatives of the vertebrata.

* The anterior end of the notochord in the developing rabbit or dog lies
where the middle of the basisphenoid bone is destined to be.

Section 3. Figure 1, Sheet 19, shows the general anatomy of
Amphioxus. We recognise four important points of resemblance to the
earlier phases of the higher and the permanent structure of the lower
members of the vertebrata, and it is these that justify the inclusion of
amphioxus in this volume. In the first place there is the--

-Notochord_.
In the next, just above it (at s.c.) we find--

-A Dorsal Tubular Nervous Axis_,
the spinal cord. Thirdly, the pharynx (ph.) is perforated by--

-Respiratory Slits_,
though these, instead of being straight slashes, are modified from
a U-shape [slant very much forward and are much more numerous than
in any true vertebrate.]. -And-, Fourthly, there is, as we shall
see, a--

-Vertebrate Type of Circulation_.
[And finally the body-wall muscles are divided into--]

[-Myomers_.]

Section 4. The alimentary canal of Amphioxus commences with an
"oral cavity," not represented in our vertebrata, surrounded by a
number of cirri, or tentacles, supported by a horny substance which
seems to be chitin, a common skeletal material among invertebrates.
A velum (v.) forms a curtain, perforated by the mouth and by two
smaller hyoidea