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 thorax, 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 ceplialotliorax. Each 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 antennae and are feelers. At the 

 base of the antennae are' the green glands, said to have renal func- 

 tions. Both the antennules and the antennae 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 niax- 

 illcv and the maxillipeds, 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, etc. The 

 food taken into the mouth passes into the stomach through a 

 short passage. The stomach is divided into two parts. The front 



