c region of Crustacea it is not the whole
segment with part of the carapace which corresponds to a vertebra, but
merely the part round the ventral nerve-cord (endophragmal skeleton).

If the skeleton of the segment in Articulates corresponds to the body of
a vertebra and is here external, then the appendages of the Articulate
must correspond to ribs (p. 538). The full development of this thought
is found in a Memoir of 1822, "Sur la vertebre."[92] He takes as the
typical vertebra that of a Pleuronectid, probably the turbot. His
original figure is reproduced (Fig. 2).

[Illustration: FIG. 2.--"Vertebra" of a Pleuronectid. (After Geoffroy.)]

He includes as part of the vertebra not only the neural (e', e'') and
haemal (o', o'') arches, but also, above and below these, the radialia
(a'', u') and the fin-rays (a', u''). (Neither the radialia nor the
fin-rays are, by the way, in the same transverse plane as the body of
the vertebra). Every vertebra, he considers, contains these nine
pieces--the cycleal (or body), the two perials (e', e'') and the two
epials (a', a'') above, the two paraals (o', o'') and the two cataals (u',
u'') below. The epials and the cataals are in reality paired bones which
in fish mount one on top of the other to support the median fins. In the
cranial region--the skull is formed of modified vertebrae--the epials
and perials open out so as to form the walls and roof of the brain; in
the thoracic region the paraals and cataals reach their maximum of
development and perform the same service for the thoracic organs, the
paraals becoming vertebral, and the cataals sternal, ribs.

We have seen that in Arthropods the body of the vertebra (cycleal) forms
the open ring of the segment, which lies immediately under the skin, the
vertebral tube coinciding with the epidermal tube. The homologues of the
other eight pieces of the vertebra must accordingly be sought in the
external appendages. At first sight there seems here a contradiction of
the principle of connections, for the appendages in Arthropods are
lateral, whereas the paired bones of the vertebra are dorsal and
ventral. But there is in reality no contradiction, for "what our law of
connections absolutely requires is that all organs, whether internal or
external, should stand to one another in the same relations; but it is
all one whether the box (_coffre_) that encloses them lies with this or
that side on the ground. What similarities in the organisatio