Arachnida — Crustacea 369 



that the primitive Arachnids arose from the Arthropod 

 branch before an aquatic had been exchanged for a ter- 

 restrial life, and that the tracheal air-tubes in Insects, 

 Centipedes, and Millipedes on the one hand, and in the 

 higher Arachnids on the other, must be regarded as 

 independently acquired (208). There is fossil evidence 

 for the existence of marine arachnids (Eurypterids) 

 in Ordovician, and of Scorpions in Silurian, times, 

 proving conclusively that for the common origin of 

 Insects and Arachnids we must go back to an im- 

 mensely remote period. 



Crustacea. — It has already been admitted (p. 351) 

 that, while the immediate progenitors of insects were 

 air-breathing animals their more remote ancestors 

 must have lived in the water. We have just seen 

 that there is undoubted evidence for a true relation- 

 ship between Insects and Arachnids, and that the latter 

 class in all probability had a marine origin. There is 

 another great class of arthropods, the Crustacea — 

 including the Lobsters, Crabs, Barnacles, Water-fleas 

 and their allies — to be considered, which is almost 

 entirely aquatic. Can any clear relationship be estab- 

 lished between these animals and insects ? A likeness 

 between certain insect-larvse and the immature forms of 

 Crustacea was at one time believed to indicate affinity 

 between the two classes ; but the view does not now 

 find much acceptance. It is generally admitted that all 

 Arthropods must be traced back to segmented worms, 

 but it is believed by many students that the Crustacea 

 have come off independently of the other classes, and 

 that the structural features which characterise arthro- 

 pods as a whole — the chitinised skin, the jointed limbs, 

 the reduced coelom, the heart with paired openings, the 

 large fore- and hind-guts — have been independently 

 acquired by the air-breathing and aquatic groups (212). 



These characters however are too fundamental to 



2 A 



