214 NATURAL HISTORY. 



effectually. All the feet (save the little pair in front of the mouth) act as jaws, and they all have 

 nippers or pincers at their extremities. The limbs of the thorax are converted into broad plates 

 covering the ovaries and gills, and we find the last pair of feet are furnished with brooms with 

 which to keep these delicate organs clean. Its eyes are placed upon the upper and anterior sur- 

 face of the great shield-shaped cuirass or carapace, and it is furnished both with compound eyes, 

 which resemble those of a Trilobite in form and position, being placed on each side of the head-shield, 

 and also with a pair of larval ocelli or simple eyes placed just in the front of the head-shield. 



Dr. S, Lockwood writes " The King Crab delights in moderately deep water, from two to six 

 fathoms. It is emphatically a burrowing animal, living literally in the mud, into which it scoops 

 and gouges its way with great facility. The anterior edge of its enormous cephalic shield is not 

 unlike in form to a cheese-cutter. The upper shell of the animal is composed of three parts the 

 forward shield, which is greatly larger than the posterior shield, and the long bayonet-shaped spine or 

 tail. In the burrowing operation the forward edge of the anterior shield is pressed downward, and 

 shoved forward, the two shields being inflected, and the sharp point of the tail presenting the 

 fulcrum as it pierces the mud, while underneath the feet are incessantly active, scratching up 

 and pushing out the earth on both sides. There is a singular economy of force in this excavating 

 action, for the alternate doubling up or inflecting, and straightening out of the two carapaces, with 

 the pushing purchase exerted by the tail, accomplish both digging and subterranean progression. 

 The Limulus is carnivorous, its food being the soft nereids or sea-worms. The King Crab has six 

 pairs of feet ; the extreme anterior pair are called antennae, being greatly shorter than the others. 

 Of the four pairs between this pair and the last pair, the basal joint of each limb is flattened 

 and smooth on each side, as though they were a series of plates intended to work upon each other. 

 The external edge of each is rounded, and bevelled like a carpenter's chisel. Thus these flattened 

 haunches lie against each other, their rounded edges directed backward at a considerable angle. 

 The bevelled edges of these projections are covered with very sharp incurved spines, overhanging 

 and pointing into the oral aperture, for it is between these five pairs of spine- clad haunches that 

 the creature's mouth is situated. These, then, are the true jaws of the animal's mouth, and as 

 there are five pairs of these manducatory joints, the creature's mouth is set in a line between ten 

 joints. These spiny teeth have, by their articulation, an amoiint of mobility in their little pits 

 which is eminently serviceable. Of these chewing teeth an individual can scarcely have less than 

 one hundred and fifty." 



It is extremely interesting to notice the occurrence at the present day of two living species of 

 Limulus, one confined to the Moluccas and to the coast of China, the other to the eastern shores 

 of North America, having continuous land separating them from each other from Tierra del Fuego to 

 the Strait of Magellan. It speaks of the great antiquity of this genus, which has survived vast 

 changes in the present configuration of land and sea, more even than is involved by the subsidence of 

 the Panama Isthmus. 



ORDER VI. EURYPTERIDA* (EXTINCT). 



ORDER YII TRILOBITAt (EXTINCT). 



The sixth and seventh orders the Eurypterida and the Trilobita are both extinct, and have 

 not been found, even in a fossil state, in any rock of younger age than the Carboniferous Limestone. 



The Eurypterida are nearly related to the King Crabs, but the body-segments are distinct, not 

 soldered together, as in Limulus ; but in both Limulus and Eurypterus the limbs serve the double 

 office of jaws and feet, being masticating organs at one end and clawed feet at the other. 



The Trilobites form one of the oldest groups of fossils known. Superficially, they closely 

 resemble the living Isopods ; but they have often more, and sometimes fewer, than seven free 

 segments between the head and tail a number nearly constant among the Isopods. 



The appendages, too, of the Trilobites appear to have been quite different from those of Isopods. 



* Greek, eurus, broad, and pteron, a wing (broad-wing), in allusion to the feet and to certain parts of Pterygotus, 

 snpposed by Louis Agassiz to have belonged to scaly nshes. 



t Greek, trilobos, three-lobed, so named because all the segments of the body are corrugated, like a piece of iron or 

 ?.inc roofing, into three arches. 



