246 MARINE INVERTEBRATES 
perfect adaptation to many different conditions of life. There is 
no class in the animal kingdom which presents so wide a range 
of organization, or whose structure deviates so widely from the 
type form. These differences lie chiefly in the external charac- 
ters and in the structure of the appendages. 
Some species are solitary, like lobsters and crabs; others, like 
shrimps, are gregarious and live in immense shoals. In the 
habits of these animals there is much which excites wonder and 
interest. 
ANATOMY OF THE HIGHER CRUSTACEA 
The body of a crustacean is divided into segments, which are 
sometimes distinctly separated, like joints, and sometimes fused 
into one piece. The head part is called the cephalic portion, the 
middle section is the thoraz, and the posterior part is the abdomen. 
The horn-like covering of the thorax is the carapace; where it 
projects over the head it is the rostrum. When the head and 
thorax are united, as in most Crustacea, the anterior or front por- 
tion of the animal is called the cephalothorax. Hach segment has 
a pair of appendages. Each appendage has a joint attached to 
the body; from this arise two jointed branches, the endopodite 
and the exopodite, the inner and the outer foot. The appendages 
are modified to perform special functions. In front of the mouth 
are two pairs: the smaller ones are the antennules and have ears 
at the base; the larger ones are antenne and are feelers. At the 
base of the antenne are the green glands, said to have renal func- 
tions. Both the antennules and the antenne are fringed with 
hairs, which aid in the sense of touch and perhaps of smell. On 
each side of the mouth are the mandibles. Then come the maz- 
ille and the mawillipeds, used in capturing and tearing the ‘food 
and conveying it to the mouth. Thus it will be seen that some of 
the appendages around the head are connected with the senses, and 
others are used in eating. Those of the thorax are for walking; 
those of the abdomen for swimming, guarding the eggs, ete. The 
food taken into the mouth passes into the stomach through a 
short passage. The stomach is divided into two parts. The front 
