660 200LOGY SECT. 



pair becoming pre-oral in course of growth, and the ganglia 

 belonging to it coalescing with the brain. The upper lip between 

 the bases of these appendages is similarly developed in both. 

 The pair of processes situated behind the sixth pair of appendages, 

 which in Limulus form the chilaria, are represented in the Scorpions 

 by a small pentagonal plate in front of the operculum. The 

 abdomen of Limulus corresponds to the pre- and post-abdomen of 

 the Scorpion ; it contains only eight segments, inclusive of the 

 telson ; but there is evidence, from a comparison with certain 

 fossil forms, that the telson represents several united metameres. 

 A certain amount of correspondence is also traceable in the appen- 

 dages of the abdomen. In both the first pair form the operculum ; 

 in the Scorpion the second pair form the pectines, while -the rest 

 disappear ; in Limulus all persist as the lamelliform appendages 

 to which the book-gills are attached. In structure there is con- 

 siderable similarity between the book-gills of Limulus and the 

 book-lungs of the Scorpion, but how far they are equivalent to 

 one another remains doubtful in view of the difference in their 

 position, the book-gills being attached to the dorsal surface of the 

 abdominal appendages and the book-lungs sunk within the 

 segments. 



The presence in both of the large " liver," the presence of a circum- 

 cesophageal artery, of a cartilaginous endosternite, and of a pair 

 of coxal glands on the basal joints of the fifth pair of appendages, 

 are some of the points of correspondence in the internal anatomy. 



While Limulus is thus closely related to the Scorpions on the 

 one hand, it exhibits, on the other, indications of affinities with the 

 Trilobites, a group of extinct Arthropods probably finding their 

 nearest existing allies in the Branchiopod Crustacea (p. 556). This 

 resemblance to the Trilobites is most marked in the stage the 

 trilobite-stage in which the young King-crab escapes from the 

 egg. Certain fossil representatives of the Xiphosura come still 

 nearer to the Trilobites than the adult Limulus, and thus increase 

 the probability that there is a genetic connection between the two 

 groups. 



It seems probable that the air-breathing Arachnida were 

 derived through Limulus-like ancestors from primitive Crustacea, 

 and that the tracheae were developed without genetic relationship 

 with those of the other air-breathing groups perhaps as modifica- 

 tions of the pulmonary sacs, the latter having been originally 

 derived from gills like those of Limulus. That air-tubes can be 

 developed in air-breathing members of what are, fundamentally, 

 aquatic groups is shown by the case of certain terrestrial Isopoda 

 among the Crustacea (p. 580). 



There is a very evident close relationship between the Myriapoda, 

 i.e., the Progoneata, and the Insecta. The Insects are more highly 

 specialised, and have their structure modified in adaptation to a 



