CLASSIFICATION OF ARTICULATA. 83 



want of distinction between thorax and abdomen ; by the equal- 

 ity of the segments of the body, the head, however, being very 

 distinct ; by the large number of legs, of which there are seldom 

 less than twenty-four pairs ; and by their aerial respiration. 



III. ARACHNIDA, including the Spiders, Scorpions, and Mites ; 

 characterised by the division of the body into two distinct por- 

 tions, the cephalo-thorax (made up of the head united to the 

 thorax) and abdomen ; by the possession of four pairs of legs ; 

 by their want of antennas ; and by their aerial respiration. 



IV. CRUSTACEA, or Crabs, Lobsters, &c. ; distinguished by 

 their aquatic respiration ; and by the possession of from Jive to 

 seven pairs of legs ; the body sometimes divided nearly as in 

 Insects, sometimes even more concentrated than in the Arach- 

 nida, and sometimes formed on the plan of that of the Myriapoda. 



658. In the Second division of Articnlata, characterised by the 

 absence of articulated members, we meet with but three classes. 



V. ANNELIDA, the Leech and Worm tribe ; characterised by 

 the extension of the body into numerous segments, which pre- 

 sent scarcely any differences from each other ; by the presence 

 of a distinct circulating system, and of respiratory organs ; and 

 by the possession of a well-developed nervous system. 



VI. ENTOZOA, or Intestinal Worms, in which the Articulated 

 form is presented in a still more degraded state, being in fact 

 sometimes scarcely recognisable ; special organs of circulation 

 and respiration are for the most part wanting ; and the nervous 

 system is altogether imperceptible or very indistinct, never pre- 

 senting the double ventral cord in a well-developed condition : 

 these have very frequently no distinct head. 



VII. ROTIFERA, or Wheel- Animalcules, a group of minute 

 animals, in which the Articulated structure is often very indis- 

 tinct, and which are especially characterised by the possession of 

 cilia, or little hair-like filaments, arranged in rows in the neigh- 

 bourhood of the mouth, and serving by their movements, both 

 for the acquirement of food, and for the aeration of the fluids of 

 the body. 



