ANNULOSA I ARACHNIDA. 259 



with the cephalothorax into a single mass. Respiration is 

 effected by tracheae. Most of the Acarina are parasitic, and 

 the most familiar are the Mites and Ticks. 



Family i. Linguatulina or Pentastomida. The members of 

 this family are singular vermiform animals, found as parasites 

 in the frontal sinuses and lungs of some Vertebrates. In their 

 adult condition they possess no external organs except two 

 pairs of hooks, representing limbs, placed near the mouth. 

 They thus closely approximate to the Tceniada, beside which 

 they have been generally placed. In the young condition, 

 however, they possess four articulated legs, and even in the 

 adult state the characters of the nervous system are higher 

 than those of the Scolecida. There are no differentiated organs 

 of respiration, and there are no circulatory organs, but the 

 sexes are distinct. 



Family 2. Macrobiotida (Tardigrada or Arctisca). The 



Fig. 99. Arachnida. a Pycnogon um lit tor ale : b Tetranychus telarius, one of the 

 " Sociable " mites ; c Hydrachna globulns, one of the " Water-mites." 



"Sloth" or "Bear animalcules," which compose this family, 

 are microscopic animals, very much like Rotifers, found in 

 damp moss and in the gutters of houses. The nervous system 

 consists of four ganglia, and there is a suctorial mouth, with 

 rudimentary jaws or stilets. The abdomen is undeveloped, 

 and there are four pairs of rudimentary legs. They exhibit 

 no traces of either circulatory or respiratory organs, and the 

 sexes are united in the same individual. 



Family 3. Acarida. This family includes the Mites, Ticks, 

 and Water-mites, some of which are parasitic, whilst others 

 are free, and some are even aquatic in their habits. The mouth 

 is formed for suction. There is no definite line of demarcation 

 between the unsegmented abdomen and the cephalothorax. 



In the true Acari (fig. 99, ), of which the Cheese-mite may 

 be taken as an example, there are four pairs of legs, adapted 

 for walking, and the mouth is provided with distinct mandibles. 



