45 8 



THE POPULAR SCIENCE MONTHLY. 



plied to the gill-chamber by the open mantle ; but, by the contraction 

 of the latter, the water is forcibly expelled through a tube termed the 

 "funnel." This expulsion of water from the branchial chamber is 

 their chief means of locomotion. 



Aerial Organs of the Skin. Special organs developed from the 

 outer surface of the body and adapted to air-breathing are confined to 



BM Iff 



Fig. 14. Spiracle op Cockchafer-Grub. 



Fig. 15. Spiracle or Flt. 



the articulates and mollusks. None are found in the radiates, this 

 great division being wholly aquatic. 



Although the crustaceans always possess gills, and are theoretically 

 classed as water-breathers, yet certain species live entirely on land, 

 and some are even killed by long immersion in water. The gill-cham- 

 bers of the land-crabs are proportionately very large, exposing more 

 surface to the ah', and the openings are small to prevent evaporation. 



Some species inhabit the highest ground of 

 West India islands, but they seek damp or 

 sheltered places, and possess some means of 

 keeping the gills moist, either by carrying a 

 small quantity of water, or by a secretion of 

 the sponge-lining of the chamber. Structur- 

 ally these organs are gills, but functionally 

 they are true lungs. 



Another crustacean, the wood-louse ( Onis- 

 cus), lives in damp places, and breathes air by 

 foliaceous gills beneath the abdomen. 



The only groups of articulates wholly air- 

 breathing are the myriapods, insects, and 

 arachnids. The myriapods show a transition from the skin-respiration 

 of the leech and earth-worm to the air-tubes of the insect. The latter 

 possesses the most peculiar and effective system for aeration. As in 



Fig. 16. Spiracle Leather- 

 Coat. 



