. ANNULOSA : ARACIINIDA. 239 



Order I. Podosomata {J^antopoda). — 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. The commoner forms of the Podosomata 

 (such as Nymphon and Pycnogonum) may be found on the sea- 

 coast at low water, crawling about amongst marine plants or 

 hiding beneath stones. Some species of the latter genus are 

 asserted to be parasitic upon fishes and other marine animals, 

 but the common British species {P. liitorale) is free when' 

 adult, and does not appear to be parasitic at any stage of its 

 existence (fig. 78, a). The legs consist of four pairs, some- 

 times greatly exceeding the body in length, and sometimes 

 containing caecal prolongations of the digestive cavity for a 

 portion of their length. The mouth is provided with a pair of 

 "chelicerae," 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. 

 Though there are no respiratory organs, there is a distinct 

 heart. The sexes are in different individuals. 



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 tracheas. 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 Scokcida. There are no differentiated organs 

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

 sexeS"aa« distinct 



Family 2. Macrobiotidce {Tardigrada or Arctisca). — The 

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

 are microscopic animals, very much hke 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 



