170 PRIMITIVE FORMS OF LIFE 



are alike, and as they are variable in number we have no 

 alternative but to connect the Myriapods with the lower 

 Crustaceans or Entomostraca, a type which, though aquatic, 

 is quite distinct from that of the Merostomata, which came 

 later. In these, however, a tracheal apparatus develops, very 

 much like that of the Arachnida, in which the respiratory 

 orifices are also close to the limbs, one pair for each segment, 

 except in the Scutigeridae, in which there are only seven 

 orifices, placed on the median dorsal line of the body. The 

 Myriapods, in short, represent the third land invasion of the 

 Arthropods, and their respiratory apparatus, in spite of 

 resemblances to that of Peripatus and the tracheate Arachnida, 

 has been formed independently and quite contrary to the old 

 adage "Nature never repeats herself". 



The Insects constituted a fourth wave of immigration, 

 undertaken not, however, by the Entomostraca with bodies 

 made up of a number of segments varying from type to type, 

 but by the Malacostraca or higher Crustaceans, which include 

 Wood-lice at one end of the scale and Crayfish at the other, 

 and mounts up through miniature freshwater Shrimps 

 and marine Shrimps to arrive finally at the Crabs. These 

 Crustaceans are innumerable, but all of them have twenty-one 

 body-segments. The Wood-lice and some related forms reached 

 the land without losing any of the characters of the Isopod 

 Crustaceans, and small tubes, elementary short tracheae, 

 develop on the respiratory feet borne on the abdomen. The 

 related Asellidae migrated to the fresh water without undergoing 

 any important modification, and there are in subterranean 

 waters certain other forms, manifestly marine in origin, since 

 species of the same genera still exist in the sea. In the same way 

 the freshwater Shrimps (Gammarus) belonging to the 

 Amphipod group, Palceomonetes, Palceomonella, and Caridina, 

 which are almost Shrimps, and Telphusa, which are Crabs, 

 all penetrated into fresh waters, and certain Crabs, of the genus 

 Birgus, and Gecarcinidae, which are Decapods, even became 

 terrestrial. But these are only individual immigrations, so to 

 speak, of relatively recent date, like the forms of the creatures 

 which accomplished them. Unquestionably such migrations 

 are still taking place. They have altered nothing in the general 

 economy of Nature. 



