I xi PHYLUM ARTHROPOD A 677 



glands on the basal joints of the fifth pair of appendages, are some 

 I of the points of correspondence in the internal anatomy. 



While Limulus is thus closely related to the Scorpions on the 

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

 I Trilobites, a group of extinct Arthropods probably finding their 

 I nearest existing allies in the Branchiopod Crustacea (p. 563). This 

 I resemblance to the Trilobites is most marked in the stage the 

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

 I egg. Certain fossil representatives of the Xiphosura come still 

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

 I the probability that there is a genetic connection between the two 



groups. 



It seems probable that the air-breathing Arachnida were 

 I derived through Limulus-like ancestors from primitive Crustacea, 

 I and that the tracheae were developed without genetic relationship 

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

 I tions of the pulmonary sacs, the latter having been originally 

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

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

 I aquatic groups, is shown by the case of certain terrestrial Isopoda 

 I among the Crustacea (p. 596). 



There is a very evident close relationship between the Myriapoda 

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

 I specialised, and have their structure modified in adaptation 



to a special mode of locomotion, but the resemblances in many 



ivspects are very strong. One of the most striking points 

 I of difference is the indefiniteness in the number of the segments 

 Jin the ^yriapoda, and their constant and definite arrangement 

 I in the Insecta. The well-defined thorax of the Insects is wanting 

 I 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 antennae 

 ; and jaws is an important point of resemblance ; so is the absence 

 | in both of the voluminous " liver " of the Crustacea and Arachnida. 

 t The gap between the two classes is narrowed by two converging 

 I groups the Symphyla among the Myriapoda on the one hand, 

 ' and the wingless and in other respects primitive Aptera among 

 [. the Insecta on the other. 



While the Insecta thus appear to be nearly related to the 

 I Progoneata, there are indications of relationship between the 



Opisthogoneata and the Onychophora, and, through these, the 

 IChaetopoda. The elongated, homonomously segmented body, the 

 j well-defined head with its antennas, the occurrence of similar 

 'appendages on all the body- segments, all point in this direction. 

 | Accordingly, instead of placing the branchiate Arthropoda 



in one group and all the air-breathing forms in another, and 

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