xi PHYLUM ARTHROPODA 677 



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. 563). 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 

 Avith 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. 596). 



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 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 — Aptera among 

 the Insecta on the other. 



While the Insecta thus appear to be nearly related to the 

 Progoneata, there are indications of relationship between the 

 Opisthogoneata and the Onychophora, and, through these, the 

 Chaetopoda. 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 of placing the branchiate Arthropoda 

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

 VOL. I xx 



