138 INVERTEBRATE ANIMALS, 
CHAPTER XIV. 
CRUSTACEA. 
Cass I. CrustaceEA.—The members of this class are com- 
monly known as crabs, lobsters, shrimps, prawns, king-crabs, 
barnacles, acorn-shells, wood-lice, &c. They are nearly allied 
to the succeeding class of the Arachnida (spiders and scor- 
pions), but are distinguished by their adaptation to a more or 
less purely aquatic life, by having jginted appendages upon the 
hinder segments of the body (abdomen), and by the possession 
of two pairs of antennz. Asa class, the Crustacea are distin- 
guished by being usually furnished with branchiz or respira- 
tory organs, adapted for breathing air dissolved in water, by 
having more than four pairs of legs, by having a well-developed 
chitinous or partially calcareous “crust” or external skeleton, 
by the fact that some of the appendages are generally so modi- 
fied as to act as organs of mastication, and by passing through 
a metamorphosis before attaining their adult condition. 
The body in a typical Crustacean is composed of ¢wenty-one 
(or, according to some writers, ¢wenty) distinct segments or 
somites, placed one behind the other. These segments are 
distributed in three distinct divisions, known respectively as 
the “head,” the “thorax” or chest, and the “abdomen” or 
tail, each of which is usually regarded as being composed of 
seven segments. In very many cases, however, the fourteen 
segments belonging to the head and chest are amalgamated to- 
gether into a single mass, which is termed the “ cephalothorax,” 
thus leaving seven segments to the abdomen. It will be un- 
necessary, however, to dwell here longer upon the structure of 
the Crustacea, as the general morphology of the class will be 
given at somewhat greater length in speaking of the lobster. 
The classification, also, of the Crustacea is so complex that it 
will be as well to omit altogether the less important orders, 
