ANNULOSA: AKACHNIDA. 201 



ORDER I. PODOSOMATA (Pantopoda). The members of this 

 order, sometimes called ' Sea-spiders,' have been placed alter- 

 nately amongst the Arachnida and the Crustacea, their true 

 position being rendered doubtful by the fact, that, though 

 marine in their habits, they possess no differentiated respira- 

 tory organs. They possess, however, no more than four pairs 

 of legs, and would, therefore, appear to be properly referable 

 to the Arachnida. Some of the order, such as Nymphon, are 

 found on the sea-coast beneath stones or upon marine plants ; 

 whilst others, such as Pycnogonum, are believed to be parasitic 

 upon fishes and other marine animals, though this is denied 

 by some. The legs consist of four pairs, sometimes greatly 

 exceeding the body in length, and sometimes containing csecal 

 prolongations of the digestive cavity for a portion of their 

 length. The mouth is provided with a pair of ' cheliceree,' or 

 chelate mandibles, and with two well developed maxillary 

 palpi, behind which in the female are a pair of false legs 

 which carry the ova. The abdomen is rudimentary. 



ORDER II. ACARINA, or MONOMEROSOMATA. The members of 

 this order possess an unsegmented abdomen which is fused 

 with the cephalothorax into a single mass. Respiration is 

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

 the most familiar are the Mites and Ticks. 



Family 1. 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. 



Family 2. Macrobiotidce (Tardigrada or Arctisca). The 

 ' 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. 



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. 



In the true Acari (fig. 58), of which the Cheese-mite may 



