218 



COLLEGE ZOOLOGY 



10 inches long and weigh less than 2 pounds. 

 The west coast lobster (Panulirus) is com- 

 monly called a spiny lobster because its 

 skeleton is provided with many needle-sharp 

 spines. 



Shrimps and prawns are also decapods. The 

 edible shrimp is slender, with whiplike 

 antennae, and is about 5 cm. long. It is 

 very agile, swimming backward with quick 

 jerks of the fin at the posterior end of the 

 body. It eats both animal and vegetable 

 matter. Shrimps are constantly being preyed 

 upon by fish and other marine animals as 

 well as by man. Prawns are smaller than 

 shrimps. A common species, which is dis- 

 played in Fig. 121, lives among rock weed 

 and eel grass on muddy bottoms along the 

 Atlantic Coast from Massachusetts to 

 Florida. 



Hermit crabs (see colored frontispiece at 

 beginning of text) are famous because they 

 live in the empty shells of marine snails. As 

 they grow larger, they must move from one 

 shell into a larger one. Frequently the shell 

 is covered with coelenterates. The abdomen 

 of the crab within the shell is soft and 

 twisted to fit the coils, and one pair of 

 abdominal appendages develops into hooks 

 which anchor the body in the shell; the 

 other abdominal appendages become de- 

 generate. The right-hand pincer, which is 

 used to capture and crush its prey, is con- 

 structed so as to close the opening in the 

 shell. The other smaller pincer fills any 

 crevice in the opening left by the other claw. 

 A common one is Fagurus, which lives in 

 rock pools and shallow water along the 

 beach from Maine to South Carolina. 



Many species of crabs are edible, but one 

 species, Callinectes sapidus (Fig. 121), is 

 usually given this doubtful honor. The short 

 and broad body is about 7 cm. long and 17 

 cm. wide. The shell is dark green and the 

 feet are blue. Edible crabs are common on 

 muddy bottoms in shallow water from Cape 

 Cod to Louisiana. In California the most 

 important edible crab is Cancer magister. 

 Crabs that are considered edible in one 



locality may not be eaten in other regions. 



Spider crabs are noted for their long 

 spidery legs. The common species, Libinia 

 emarginata, is about 7 cm. long and lives 

 on mud flats and oyster beds along the 

 Atlantic Coast from Maine to Florida, and 

 in California. The shell is pointed in front 

 and covered with a dense growth of 

 chitinous "hairs" which give it a furry ap- 

 pearance. The legs of a Japanese spider crab 

 reach an enormous length; individuals of 

 this species may measure 12 feet across when 

 spread out. 



Fiddler crabs (Fig. 121) are accustomed 

 to wave their large claw back and forth as 

 though playing a violin, hence the common 

 name. They form colonies which live in 

 burrows that are dug in the mud or sand in 

 salt marshes. In certain species, the males 

 are very pugnacious and fight each other 

 with great vigor. 



ORIGIN AND RELATIONS 

 OF THE ARTHROPODA 



The Arthropoda probably evolved from 

 an annelidlike ancestor. 



The Onychophora (Fig. 124) seem to 

 resemble most closely this ancestral condi- 

 tion. They possess a thin cuticle, a continu- 

 ous and muscular body wall, no joints, one 

 pair of jaws, and appendages on the first 

 segment. In the other groups of arthropods, 

 development of a rigid exoskeleton with 

 joints brought about a change in the dis- 

 tribution of the muscles from the continu- 

 ous type forming a muscular body wall, as 

 in the annelids, to the discontinuous type in 

 which the muscles are separately and di- 

 versely developed for the movement of spec- 

 ial segments. 



The crustaceans, insects, centipedes, and 

 millipedes appear to have developed along 

 one line, since they have so much in com- 

 mon; and the arachnids along another line, 

 since none of their appendages have de- 

 veloped into antennae, and none possess 



