RELATIONSHIP OF LIMULUS TO TRILOBITES. 303 
cently hatched Limulus (Fig. 271), that of Barrande’s larva 
of Trinucleus ornatus (Fig. 272, 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 pass 
through a well-marked metamorphosis, though by no means 
so remarkable as that of the Decapods and the Phyllopods. 
Fig. 272.—Larva of a Trilo- 
bite, Trinucleus ornatus.— 
After Barrande, 
Fig. 271.—Larva of the King-crab. 
The young king-crabs swim briskly up and down, skim- 
ming about on their backs like Phyllopods, by flapping their 
gills, not bending their bodies. In asucceeding 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 appearanee from those of their living descend- 
ants. 
Limulus, Prestwichia, Bellinurus, and Fuproéps form 
