ARTHROPOD A. 229 



nallegs; (2) internal sacs (lungs) with numerous leaf- like 

 folds ; (3) air-tubes or tracheae, strikingly like those of the 

 Insecta, but with a different history. The reproductive 

 organs open near the middle of the body. 



SUBCLASS I. MEEOSTOMATA. 



Here belong the horseshoe crabs of our east coast (and a 

 number of fossil forms), which breathe by means of leaf- 

 like gills, whicli have both simple and compound eyes, and 

 which have the bases of the walking-feet of the cephalo- 

 thorax modified to serve as jaws. Recent investigations 

 show that the horseshoe crabs are not related to the true 

 crabs, but are to be rather closely associated with the 

 scorpions. These forms live in the sea, feeding on worms, 

 etc., found in the sea-bottom, coming to the shore in 

 spring and early summer to lay their eggs. The horse- 

 shoe crabs are without any economic importance, as they 

 are useless as food, but they are extremely interesting to 

 the naturalist, as they are the last remnants of forms which 

 were once abundant in the seas of past times. 



SUBCLASS II. ARACHNTDA. 



With few exceptions, the Arachnids are terrestrial forms. 

 They breathe by internal lungs or by tracheae, and they 

 lack compound eyes. There are several orders of Arach- 

 nids, but only a few of them need be mentioned here, as 

 some are inconspicuous, while others occur only in the 

 warmer regions of the globe. 



ORDER I. SCORPIO^IDA. 



The scorpions have a single pair of jaws (mandibles) and 

 a pair of large pincers, much like those of lobster or crab. 

 The long abdomen is distinctly jointed, the seven basal 



