74 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



body is either elongated and distinctly jointed, with a 

 short terminal spine, or is greatly contracted, and 

 globose in form. 



3. L.ipobraiicliiata. In these the respiratory 

 lamellae are lost, and if any special respiratory organs 

 are developed, they are in the form of tracheal tubes. 

 Here belong the Acariiia (mites, ticks), with an 

 imsegmented abdomen, and often a sucking mouth; 

 the Pedipalpi (Harvestmen), with a segmented 

 abdomen; and the Pycnogonida (no-body crabs), 

 in which prolongations from the gastric cavity extend 

 into the enormously long legs (Fig. 32). 



Appended to this group, but considerably altered 

 by parasitism, so that when adult they have elongated 

 worm-like bodies, with but two pairs of mouth hooks 

 to represent the appendages, are the Peiitastomida, 

 the best known example of which is the Pentastomum 

 taenoides, which is found in the frontal sinuses of 

 dogs' skulls (Fig. 33, A, B, c). 



C. The third division of Arthropoda is that of the 

 Traclieata ; in them there is always one pair of 

 antennae in front of the mouth, the gnathites may be 

 very profoundly modified ; respiration is effected by 

 means of air tubes (tracheae), which are regularly 

 arranged and richly developed within the body. They 

 are divisible into a lower and a higher group, of which 

 the former has comparatively few representatives ; 

 the other more than all the rest of the animal 

 kingdom. 



I. Myriopoda or Centipedes and Millipedes. 

 In these most of the metameres are separate and 

 distinct, or are united by pairs, and all are provided 

 with a pair of jointed appendages. The mouth organs 

 are not greatly modified ; they are all terrestrial. 



II. Hexapoda or Insects. In the vast assem- 

 blage of forms associated under this head, the 

 appendages of the adult are never functionally 



