454 THE INVERTEBRATA 



in under them. There are five pairs and the first of these corresponds 

 in position to 'the pectines of the scorpion (except possibly in Glypto- 

 scorpiiis). Thus, when the ancestors of the scorpions became terrestrial, 

 we may suppose that the first pair of respiratory appendages remained 

 external and took on a sensory function, while the rest helped to 

 form the lung books. 



Minute forms with incompletely developed abdomen and enlarged 

 eyes have been found which are thought to be the pelagic larvae of 

 eurypterids. The adults were in all probability carnivorous forms, 

 which crept and swam and sometimes burrowed at the bottom of 

 shallow seas. In Pterygotus (Fig. 334 A) and Eurypterus there are 

 adaptive modifications of the telson for swimming and burrowing 

 respectively. 



Class XIPHOSURA 



Aquatic arachnids with a broad prosoma divided by a hinge from the 

 opisthosoma in which the first six segments are present and fused 

 together dorsally; they bear six pairs of biramous appendages, of 

 which the first form an operculum on which the genital apertures 

 open and the remaining five carry the gill books ; chelicerae of usual 

 arachnid type, pedipalps not distinguished from the four pairs of 

 ambulatory appendages which follow; mouth far back surrounded 

 by gnathohases of all the postoral limbs ; caudal spine present possibly 

 representing the lost abdominal segments as well as the telson; 

 pregenital segment represented by rudimentary appendages, the 

 chilaria. 



Limulus (Figs. 335, 336), which is the sole living representative of 

 the group, is evidently more affected by specialization than either the 

 scorpions or eurypterids, and it is on this account that the attempts 

 which have been made to indicate the king crab as an ancestral form 

 to higher groups have usually been regarded as ingenious but illusory. 

 It is essentially a shore-living, burrowing animal. Like a crab, its 

 carapace is compact, dorsoventrally flattened and expanded laterally, 

 so that the animal can shovel its way under sand and mud. Its legs 

 are tucked under the carapace and the hinder pair kick out the sedi- 

 ment behind. To protect the gill books from this rough treatment, the 

 operculum completely covers the appendages which bear them. But 

 Limulus has not lost its tail, and an observer, watching the creature in 

 an aquarium, will contrast it unfavourably for grace and efficiency with 

 a crab. Its swimming movements, principally brought about by the 

 flapping of the abdominal appendages, are slow and clumsy, and we 

 can hardly consider it except as a sedentary animal. 



The chelicerae are small, chelate and three-jointed, as is usual in 

 arachnids. The succeeding four pairs of appendages are all alike in 



