9R 16 1907 
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SOUTHERN CALIFORNIA ACADEMY OF SCIENCES 3 
Morphology. The Arachnida are not insects, though often 
incorrectly spoken of as such; but may be defined as arthrop- 
odous (joited) animals, in which the body is never composed 
of more than 18 somites or segments, and is divisible into 2 
main regions—an anterior or cephalo-thorax, and a posterior 
or abdomen. The class comes between the Crustace and 
Hexarthra or insects, being distinguished from each by marked 
structural differences, yet having strong affinities to both. 
The chief external differences between it and the Crustace 
are: the absence of a specialized auditory organ, the simpli- 
fication of the mouth-parts, the modification of the gills into 
lung-saes, and simple instead of compound eyes; whilst the 
swhmmineg-telson of the lobster (for instance) becomes either a 
weapon, a harmless whip-tail, or is entirely absent. 
The Arachnida may be distinguished from the true insects 
by the division of the body into two instead of into three main 
regions; by the presence of 8 instead of 6 legs, and the absence 
of antenne. The eyes are simple instead of compound, there 
are no wings; and in most genera lung-saes take the place of 
the tracheex of the insect. 
In this paper I omit the remaining orders of Arachnida, 
namely: the false or book-scorpions, the Opiliones or harves- 
ters, and the Acari or ticks; nor do I deal with the orb-weaving 
spiders. 
In the Arachnida we have a type of marked persistence of 
life, for there are but slight differences between fossil and re- 
eent Arachnida; and, since no advanee in organization is indi- 
eated, persistence of habit may be inferred. These animals 
undergo no biogenetic changes, such as the larval and pupa 
stages of the insect. On the contrary, the individual is born 
in substantially the same form as that which characterizes its 
adult life; it merely imereases in size by a series of moults. 
From this we infer that ages ago the Arachnida evolved those 
specific forms best ensuring survival; though naturally suffi- 
cient time has elapsed for them to have become specialized into 
many diverse forms. 
The remote ancestor of the class was possibly one of the Che- 
topoda, or bristle-footed worms; and the archetype may have 
been a creature, of which the nearest recent representative is 
that curious and lonely survival, the Peripatus, which is neither 
worm nor insect: and over whose classification a war has been 
waged among zoologists as bitter as the historic strife in miero- 
secopieal eireles, over the relation which the numerical aperture 
of the condenser should bear to that of the objective. 
If by means of diagrams we represent the archetype of the 
Arachnida, and the specialized generic forms evolved there- 
LIBRA 
NEW Y 
BOTAN 
GARL 
