BACKBONELESS ANIMALS WHICH BREATHE IN AIR 443 



this necessitates a more complex blood-system, with better defined 

 and more numerous vessels. 



Whip-Scorpions possess two pairs of abdominal lung-books, 

 similar to those of Scorpions. This is also the case with the larger 

 Spiders, such as the bird -catching form 

 (Mygale avicularia) of South America 

 (fig. 561). The smaller species, such as 

 the familiar Garden- and House-Spiders, 

 present a modification of this arrangement. 

 They retain the front pair of lung-books, 

 but the hinder pair are replaced by two 

 sets of air-tubes, resembling in character 

 those of insects, though possibly of dif- 

 ferent origin. 



The remaining groups of Arachnids 

 possess air-tubes in those cases where 

 special breathing organs are present at all. 

 This is the case, for example, with many 

 of the small forms known as Mites, of 

 which Red "Spider" ( Tetranychus telarius] 

 is so provided. The Cheese- Mite (Tyroi- 



. x . -11 ' i 



glyphus szro) is an instance illustrating the 



/- 1 1 1 . 



aDSenCe OI Special breathing Organs. In 



, rr , , , 



such cases respiration is effected by the 



general surface of the body, as in many small animals of widely 



different kind. 



AIR-BREATHING CRUSTACEANS (CRUSTACEA) 



We have seen that the lung-chamber of an ordinary land-snail 

 (see p. 433) is really to be looked upon as a gill-cavity from which 

 the gills have disappeared, and which has acquired a new way of 

 breathing. Very much the same sort of statement can be made 

 as regards the thorough-going Land-Crabs, creatures, no doubt, 

 descended from ancestors which lived between tide -marks, and 

 gradually came to depend less and less upon the air dissolved in 

 sea- water. In such a land-crab the large gill-chamber has been 

 converted into a lung, numerous folds and outgrowths being pre- 

 sent on its lining, which offer a large surface for exposure to air. 

 At the same time the gills have been very much reduced in size, 

 and may even have practically disappeared. 



Fig. 561. Mygale (partly dissected) 

 from below 



mg-legs, cut short ; L.B., lung-books, one 



of which is cutopen to show leaflets :S T., 



openings into lung-books; M, a muscle- 

 band; N., nervous system; ov., ovary; 

 SP., spinneret. 



