_L'Organisation du regne animal_; Gaubert, "Recherches sur les
Arachnides," _Ann. Sci. Nat._ (7) vol. xiii., 1892; Koch, C., _Die
Arachniden_ (16 vols., Nuremberg, 1831-1848); Koch, Keyserling and
Sorensen, _Die Arachniden Australiens_ (Nuremberg, 1871-1890); Pocock,
_Arachnida of British India_ (London, 1900); _Idem_, "On African
Arachnida," in _Proc. Zool. Soc._ and _Ann. and Mag. Nat. Hist._,
1897-1900; Simon, _Les Arachnides de la France_ (7 vols., Paris,
1874-1881); Thorell, "Arachnida from the Oriental Region," _Ann. Mus.
Genova_, 1877-1899. (E. R. L.)

FOOTNOTES:

[1] See the article ARTHROPODA for the use of the term "prosthomere."

[2] See fig. 12 in the article ARTHROPODA.

[3] Though ten is the prevailing number of retinula cells and
rhabdomeres in the lateral eye of Limulus, Watase states that they
may be as few as nine and as many as eighteen.

[4] A great deal of superfluous hypothesis has lately been put
forward in the name of "the principle of convergence of characters"
by a certain school of palaeontologists. The horse is supposed by
these writers to have originated by separate lines of descent in the
Old World and the New, from five-toed ancestors! And the important
consequences following from the demonstration of the identity in
structure of Limulus and Scorpio are evaded by arbitrary and even
phantastic invocations of a mysterious transcendental force which
brings about "convergence" irrespective of heredity and selection.
Morphology becomes a farce when such assumptions are made. (E. R. L.)

[5] A pair of round tubercles on the labram (camerostome or
hypostoma) of several species of Trilobites has been described and
held to be a pair of eyes (22). Sense-organs in a similar position
were discovered in Limulus by Patten (42) in 1894.

[6] The writer is indebted to R.I. Pocock, assistant in the Natural
History departments of the British Museum, for valuable assistance in
the preparation of this article and for the classification and
definition of the groups of Eu-arachnida here given. The general
scheme and some of the details have been brought by the writer into
agreement with the views maintained in this article. Pocock accepts
those views in all essential points and has, as a special student of
the Arachnida, given to them valuable expansion and confirmation. The
write