Crustacea 371 



pairs. In the lower Malacostraca (Amphipods and Iso- 

 pods) the first pair of thoracic limbs become an extra 

 pair of maxillse, so that there are four pairs of true jaws. 

 According to a recent interpretation of the mouth- 

 parts of the Collembola and Thysanura (213) this 

 state of things finds an exact parallel in the jaws of 

 those two lowly orders of insects. Though this view 

 has not met with general acceptance, no doubt can be 

 thrown on the accompanying observation that the 

 mandibles of the Collembola and Thysanura, both in 

 form and muscle attachments, resemble the mandibles 

 of Crustacea more nearly than they resemble the 

 mandibles of higher Insects. In any case the near 

 and probably identical correspondence in the number 

 of segments in Insects, Arachnids, and the higher 

 Crustaceans, is too marked to be the result of chance ; 

 it must point to a far-olf common origin for the three 

 classes. And the retention of feelers, together with 

 the correspondence in the head-segments, seems to 

 show that the Insects are really nearer to the Crusta- 

 ceans than to the Arachnids. Among the air-breath- 

 ing arthropods the classes Chilopoda and Diplopoda 

 show great variation in the number of their segments, 

 while the more highly organised Insects and Arachnids 

 can be clearly traced to an ancestor with twenty. So 

 among the Crustacea, the lower orders (Entomostraca), 

 seem to have arisen by an increase or reduction in the 

 twenty segments which remain constant in the higher 

 orders. A group of Crustaceans (Leptostraca), with 

 an extra limbless segment to the hind-body, are in 

 many respects intermediate between the Malacostraca 

 and Entomostraca. This group, which had many 

 representatives in the Cambrian period, is now re- 

 duced to a few forms only. 



The probable correspondence (homology) between 

 the segments and limbs in the various arthropod 

 classes is shown in the accompanying table. 



