l894 .] THE BRITISH NATURALIST. 15 



In man and other vertebrates, the hard parts of the body and the 

 levers (for that is what the bones of our legs and arms are), which move the 

 body are internal to the muscles. The muscles are outside and cover the 

 bones. In Arthropods the opposite obtains, the muscles being" internal to 

 the body and limbs. Having pointed out the two characters common to 

 all Arthropods, it will be well if we now divide this great natural group 

 into the two sub -kingdoms which have been previously referred to, viz., 

 water-breathers and air-breathers. There is no doubt, in a sense, 

 all Arthropods are air-breathers, for even those which breathe by gills 

 extract the oxygen held in solution by the water ; but though the function 

 of respiration is of course common to both sub-kingdoms, the organs ol 

 respiration are fundamentally different. 



There are a few terrestrial crustaceans, but even these retain their 

 characteristic respiration. Their gills are, however, so placed as to lose 

 their moisture very slowly, and thus, as these animals live in damp places, 

 they are able to survive as terrestrial forms, although their breathing 

 organs are adapted for aquatic life. It should be remembered that 

 respiration can only be effected through a moist membrane ; the crustacean 

 gill is kept moist by the water ; in the air-breathing Arthropods the 

 tracheae, which are minute tubes ramifying within the body, are kept 

 moist by the blood. 



Just as some of the essentially aquatic crustacean forms have become 

 modified lor terrestrial life, but still exhibit in their respiratory organs 

 the indisputable marks of their ancestry ; so amongst the air-breathing 

 Arthropods, we find some adult and many larval forms which have become 

 aquatic, but retain in their new element the breathing apparatus, of which 

 structure is essentially that of their terrestrial relatives. In such forms 

 the oxygen held in solution in the water passes through a more or less 

 modified membrane into the tracheal system, and thereby reaches the 

 interior of the body. Thus, in aquatic Tracheata, as in their terrestrial 

 relatives, the air is conveyed into the interior of the body before it comes 

 into contact with the blood which it. purifies. 



Having briefly noted the general characteristics of the Arthropods, and 

 seen the fundamental difference between the two great sub-kingdoms, we 

 may proceed to a more detailed discussion of the Tracheata which are 

 ; divided into five classes. The animals comprising these five classes have 

 nothing in common beyond the general arthropod characters previously 

 described, and the more or less modified tracheal system. 



There are many other features which are nearly, but none that are 

 quite, characteristic. In classes comprising, as these do, the overwhelm- 

 ing majority of animal species, it will be evident and certain there will 

 be some exceptions to any rule. 



(To be continued . ) 



