628 ZOOLOGY SECT. 



glands on the basal joints of the appendages of the fifth pair, 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 Phyllopod Crustacea (p. 559). 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 the Crustacea, and 

 that the tracheas were developed as modifications of the pulmonary 

 sacs, the latter having been originally derived from gills like those 

 of Limulus. 



There is a very evident close relationship between the Myria- 

 poda and the Insecta. The Insects are more highly specialised, 

 and have their structure modified in adaptation to a special mode 

 of locomotion, but the resemblances in many respects are very 

 strong. One of the most striking points of difference is the 

 indefiniteness in the number of the segments in the Myriapoda, 

 and their constant and definite arrangement in the Insecta. 

 The well-defined thorax of the Insects is wanting in the Myriapods 

 in general, but certain of the segments following the head differ 

 from the rest in various respects, and might be looked upon as 

 constituting a thoracic region. The presence in both groups of a 

 sharply marked-off head bearing antennas and jaws is an important 

 point of resemblance ; so is the absence in both of the voluminous 

 :< liver " of the Crustacea and Arachnida. The gap between the 

 two classes is narrowed by two converging groups the Symphyla 

 among the Myriapoda on the one hand, and the wingless and in 

 other respects primitive Apt era among the Insecta on the other. 



While the Insecta thus appear to be nearly related to the 

 Myriapoda, there are indications of relationship between the latter 

 class and the Onychophora, and, through these, the Chsetopoda. 

 The elongated, homonomously segmented body, the well-defined 

 head with its antennae, the occurrence of similar appendages on all 

 the body segments, all point in this direction. Accordingly, instead 

 <>f placing the branchiate Arthropoda in one group and all the 

 air-breathing forms in another, and deriving the latter from the 

 former, we should probably express more correctly the affinities of 

 the various groups of Arthropods by some such scheme as that 

 expressed in the diagram (Fig. 523). 



Here an intermediate link between Annelida and the existing 



