ly the first three
pairs of feet are present in _Julus_ at birth (Degeer), "an observation,
which, together with their position, should cause them to be considered
as the representatives of the six thoracic feet of Hexapoda" (p. 44).

His comparison of the Arachnid appendages with those of insects and
Crustacea is very curious. As his starting-point he takes _Cyamus_,
which has antennae (two pairs) and mouth parts (four pairs) as in many
Crustacea, and then seven pairs of legs; he compares with it _Nymphon_,
which has in all seven pairs of appendages. These appendages he
homologises with the seven pairs of legs of _Cyamus_, so that the first
appendage in _Nymphon_ corresponds to the seventh appendage of _Cyamus_.
This homology is extended to all Arachnids; their first two pairs of
appendages, however they may be modified as "false" mandibles and
"false" maxillae, really correspond to the second and third maxillipedes
in Crustacea, and to the second and third pairs of feet in insects. It
is interesting to note that he treats _Limulus_ as an Arachnid, pointing
out that there is as much difference between _Apus_ and _Limulus_ as
between _Cancer_ and _Phalangium_. He describes the "gnathobases" in
_Phalangium_ and _Limulus_. We may note that he had just an inkling of
the modern doctrine that all the appendages of Articulates consist of a
basal joint bearing an inner and an outer terminal piece, for he
observes that the "cirri" of the maxillipedes of Crustacea give the
appendage the same bifid appearance as the appendages of the abdomen and
the thoracic legs of _Mysis_ (p. 50).

V. Audouin, in his memoir, _Recherches anatomiques sur le thorax des
animaux articules_,[135] applied the principle of the unity of plan and
composition to the exoskeleton of insects, Crustaceans, and Arachnids.
His guiding ideas were, "(1) that the skeleton of articulated animals is
formed of a definite number of pieces, which are either distinct or
intimately fused with one another; (2) that in many cases, some pieces
diminish or altogether disappear, while others reach an excessive
development; (3) that the increase of one piece seems to exert on the
neighbouring pieces a kind of influence which explains all the
differences one finds between the individuals of each order, family and
genus" (Sep. copy, p. 16). Geoffroy had already stated, without proof,
that the parts of the Arthropod's skeleton, however they might change in
shape and size, remain