The members of this order possess an unsegmented abdomen which is fused with the cephalothorax into a single mass. Respiration is effected by tracheae, or by the integument. Most of the Acarina are parasitic, and the most familiar are the Mites and Ticks.

Family 1. Pentastomida (Linguatulinae)

The members of this family are worm-like parasites, which in their adult state are found in the interior of the frontal sinuses, the nose, or the lungs of the Dog, and of other Vertebrate animals. When fully grown (fig. 163) they are completely vermiform, with a soft annulated integument, and possessing no external organs except two pairs of retractile hooks, representing limbs, placed near the mouth. The adult thus presents an external resemblance to the Taeniae, from which, however, they are separated by the details of their internal organisation. There are no differentiated organs of respiration or circulation, but the sexes are distinct. The larvae (fig. 163, B) are found encysted in the liver or other internal organs of various Vertebrates (including man), and possess two pairs of articulated limbs.

Family 2. Tardigrada (Macro-biotidae or Arctisca). - This family comprises the so-called "Sloth" or "Bear Animalcules," which are microscopic animals found in damp moss and in the gutters of houses (fig. 165, B). In form, the body is somewhat vermiform, with four pairs of rudimentary legs. The mouth is suctorial, with rudimentary jaws or stylets. They exhibit no traces of respiratory or circulatory organs, and, unlike the other Arachnids, they have the sexes united in the same individual.

Fig. 163.   A, Pentastoma tamoides, female, of the natural size; C, Male of the same, of the natural size ; B, Larva of the same, greatly enlarged, showing the two pairs of articulated limbs. (After Spencer Cobbold and Leuckart.)

Fig. 163. - A, Pentastoma tamoides, female, of the natural size; C, Male of the same, of the natural size ; B, Larva of the same, greatly enlarged, showing the two pairs of articulated limbs. (After Spencer Cobbold and Leuckart.)