Chapter IV: Section II: Entomostraca

Shell not solid; legs variable in number; mouth variable.

ORDER 6. _Branchiopoda._ Integuments horny, branchiæ feathery,
forming part of the feet.

It is to this division of the Crustacea that the Cirrhopoda are now
referred.

ORDER 7. _Pæcilopoda_, mouth suctorial.

Sub-order 1. _Xiphosura_, or king-crabs.

Sub-order 2. _Siphonostoma_, or fish parasites.

III. _The Arachnida_ are defined by Lamarck to be oviparous
animals, provided with six or more articulated legs, not subject to
metamorphosis, and never acquiring any new kinds of organs. It is
now known, however, that some mites undergo a sort of
metamorphosis, having only six legs when first hatched, and passing
through a quiet pupa stage before acquiring their perfect form.
Their respiration is either by means of air-sacks, which serve for
lungs, or of a kind of tube with circular openings for the
admission of air. There is a rudimentary heart and circulation in
most of the species. There are two orders; those with lungs, and
those without.

ORDER I. _Pulmonariæ._ The Arachnides comprised in this division
have air-sacks, which serve for lungs, a heart with distinct
vessels, and from six to eight simple eyes. There are two distinct
families: viz. _Araneides_, comprising all the spiders and
spinners; and Pedipalpi, comprising the tarantula and scorpions.

ORDER II. _Tracheariæ._ These Arachnides are distinguished by their
respiratory organs, which consist of radiated or branched tracheæ,
receiving air by two circular openings. Their eyes vary from two to
four. The principal animals belonging to this division are the
long-legged spiders (_Phalangium_), and the mites (_Acarus_),
including the gardener’s pest, the little red spider (_Acarus
telarius_), the cheese mite (_Acarus Siro_), and the harvest bug
(_Acarus_ or _Leptus autumnalis_).

IV. _The Insecta_ form the fourth and last class of articulated
animals, and they derive their name from the Latin word _insectum_,
which signifies “cut into,” in allusion to the distinct divisions
of head, thorax, and abdomen in the true insects: and in
contradistinction to the Annelides, the bodies of which present no
such divisions. The true insects are defined as animals without
vertebræ, possessing six feet, with a distinct head furnished with
antennæ, and breathing through stigmatic openings, which lead to
interior tracheæ. The Myriapoda have, however, more feet. The
following are the twelve orders into which this class is divided.