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, remained faithful to the principle of connections, at least at their points of insertion.[137] Audouin gave the detailed demonstration of this by his accurate and minute determination of the pieces of the arthropod skeleton. He recognised that the body of Arthropods was made up of a series of similar rings, and that even the compact head of insects consisted of fused segments. In each segment Audouin distinguished a fixed number of hard chitinous parts, the dorsal tergum, the ventral sternum, the lateral "flanc" of three pieces, all to be recognised by their positions relative to one another. Many of the names which he proposed are still in use; it was he who introduced the terms prothorax, mesothorax, and metathorax, for the three segments of the insect's thorax.