xviii Introduction. 
covered with a crust or shell, divided into segments by trans- 
verse incisions, the segments being united by a strong mem- 
brane. Once a year the larger species of these animals moult, 
throwing off their old crust or shell, and forming a new one, 
the animal remaining in a naked and greatly weakened state 
during the intermediate time. Many of the Crustacea swim 
with great ease, but on land their motions are generally cramped 
and awkward ; and they are confined to crawling, or leaping by 
means of the tail. When a limb is injured they possess the ex- 
traordinary power of throwing it off, and forming a new one. 
The Crustacea lay eggs, and the young of some of the species 
undergo a transformation before they attain their full size. The 
Crustacea were divided into two sections and seven orders by 
Latreille, which are as follow : — 
Section I. Malacostraca. 
Shell solid, legs ten or fourteen, foot-jaws six or ten, mandibles two, 
maxillae four ; mouth with a labrum. 
Sub-section I. Podophthalma, eyes on foot-stalks. 
Order 1. Decapoda, legs ten. 
Sub-order 1. Brachyura y the crabs. 
Sub-order 2. Macroura, the lobsters. 
Order 2. Stomapoda, legs more than ten. 
Sub-section 11. Edriophthalma, eyes not on foot-stalks. 
Order 3. Amphipoda, body compressed ; mandibles palpigerous. 
Order 4. Lcemodipoda, abdomen rudimental, with only the rudiments 
of one or two pairs of appendages. 
Order 5. Isopoda, body depressed ; abdominal appendages flat ; man- 
dibles not palpigerous. 
Section II. Entomostraca. 
Shell not solid ; legs variable in number ; mouth variable. 
Order 6. Branehiopoda. Integuments horny, branchiae feathery, form- 
ing part of the feet. 
It is to this division of the Crustacea that the Cirrhopoda are now 
referred. 
Order 7. Peecilopoda, 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 meta- 
morphosis, and never acquiring any new kinds of organs. It is 
now known, however, that some mites undergo a sort of meta- 
morphosis, 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. Puhnonarice. 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 
