510 The Animal Kingdom 



vertebrate forms. At present there are no living species, but their fossil 

 remains are widely distributed throughout sedimentay rocks. 



There were three body regions : the head, thorax, and pygidiiim. 

 The flattened body was divided into three lobes by longitudinal fur- 

 rows, and the pygidium was formed by the union of the posterior seg- 

 ments. The head region was but poorly developed and had but a single 

 pair of antennae and in most cases a pair of compound eyes. The head 

 was covered by a shieldlike carapace. Except for the last segment, each 

 body segment had a pair of biramous appendages. 



These forms had a free-swimming larval stage which bore re- 

 semblance to the nauplius larva of the crustaceans. 



THE CLASS MEROSTOMATA 



The members of this class are usually considered to be closely re- 

 lated to the spiders and allied groups due to the similarity of the ap- 

 pendages. In past ages, this was an abundant group, but at present 

 there are but four known species which at times are found in quite large 

 numbers. One of these, the king crab, Limulus polyphemus is found 

 from Newfoundland to Mexico; the others are found in the Orient. 

 Often these are called the "horseshoe crabs" due to the resemblance of 

 the carapace to a horseshoe ; actually, of course, this animal is not a 

 crab and is not closely related to the crustaceans. 



The cephalothorax dorsally bears lateral compound eyes and ven- 

 trally has six pairs of appendages. These appendages include the three- 

 jointed chelicerae, and the six- jointed legs. There is a wide, unseg- 

 mented abdomen and a long pointed terminal telson. Ventrally the 

 abdomen has six pairs of thin, broad appendages, the last five of which 

 function as gill books. The "leaves of these books" are well supplied 

 with blood vessels for respiration ; the sexes are separate. 



Limulus lives in the shallow coastal waters, burrowing in the sand 

 and mud and feeding on worms and other small invertebrates. In the 

 early summer, it goes to sand beaches, mates, makes a depression in the 

 sand, and lays the eggs. The larva that hatches from the egg resembles 

 very closely the adult ancient trilobites and is known as a trilobite larva 

 (Fig. 170,C). Limulus may grow to one foot in length, not including 

 the telson. As it grows, it casts its exoskeleton in typical arthropod 

 fashion and these are often found washed up in large numbers along the 

 shores. 



