Crustacea. 85 



2. But the limy material is not deposited everywhere in 

 the cuticle. Certain places are left for joints. At these 

 places the chitin remains flexible. Hence we have a series 

 of segments joined together, five in the head, consolidated, 

 eight in the thorax, and usually seven in the abdomen, 

 twenty being the typical number in the higher forms. 



3. Not only is the body segmented, but normally each 

 segment bears a pair of appendages, which are themselves 

 segmented. The eyes are no longer regarded as append- 

 ages, but outgrowths of the head, which later become 

 movable by means of a joint. 



4. Most crustaceans have gills and lead an aquatic life. 

 Some of the simpler forms breathe by the whole surface 

 of the body, and a few forms which have gills live out of 

 water, but usually in damp places. The gills remain moist, 

 a small amount of water serving to transmit the oxygen 

 from the air to the blood within the gills. 



5. Crustacea normally possess two pairs of antennae. 



6. Most crustaceans have compound eyes. 



7. Crustaceans are an active group, but, as above 

 noticed, some are sessile, and others parasitic. 



The King Crab. The king crab, or horseshoe crab, has a body 

 shaped somewhat like a horseshoe. A six-sided abdomen fits into a 

 deep notch in the posterior margin of the cephalothorax, and ends in a 

 long, tapering spine. On the cephalothorax is one pair of simple and 

 one pair of compound eyes. The mouth is in the center of the under 

 surface, between the bases of the legs, and a series of leaflike gills are 

 to be found under the abdomen. The king crab is found along our 

 Atlantic coast, often burrowing in the sand. It molts by splitting the 

 shell along the anterior margin. The hard crust, the molting, the gills, 

 and general mode of life would seem to ally the king crabs to the Crus- 

 tacea, but later researches place them nearer the spiders. They have 

 some points of relationship with the extinct trilobites, and are especially 

 interesting as the only known survivors of their race. 



