RELATIONSHIP OF L1MULU8 TO TRILOBITES. 323 



cently hatched Limukis (Fig. 276), that of Barrande's larva 

 of Trinuchus ornatus (Fig. 277, natural size and enlarged). 

 He will see at a glance that the young Trilobite, born with- 

 out any true thoracic segments, and with the head articu- 

 lated with the abdomen, closely resembles the young Limu- 

 lus. In Limulus no new segments are added after birth ; 

 in the Trilobites the numerous thoracic segments are add- 

 ed during successive moults. The Trilobites thus i^ass 

 through a well-marked metamorphosis, though by no means 

 so remarkable as that of the Decapods and the Phyllopods. 



Fig. 277.— Larva of a Trilo- 

 bite, Trinudeus ornatus. — 

 After Barrande. 



Pig. 376. -Larva of the King-crab. 



The young king-crabs swim briskly up and down, ekim- 

 ming about on their backs like Phyllopods, by flapping their 

 gills, not bending their bodies. In a succeeding moult, which 

 occurs between three and four weeks after hatching, the 

 abdomen becomes smaller in proportion to the head, and the 

 abdominal spine is about three times as long as broad. At 

 this and also in the second, or succeeding moult, which oc- 

 curs about four weeks after the first moult, the young king- 

 crab doubles in size. It is probable that specimens an inch 

 long are about a year old, and it must require several years 

 for them to attain a length of one foot. 



The Limuli of the Solenhofen slates (Jurassic) scarcely 

 differed in appearance from those of their living descend- 

 ants. 



Limulus, Prestwichia, BeUinurus, and Etqjroojjs form 



