THE AEACHNIDA. 123 



The reproductive aperture and the copulatory organs of the 

 males are situated on the ventral surface of the second to the 

 seventh segments of the body. 



The AEACHNIDA are divisible into six orders 



1. The ARTHROGASTRA have the abdomen distinctly divided 

 into somites, and passing by a broad base into the cephalo- 

 thorax. Eespiration takes place in some by lung sacs, and in 

 some by tracheae. 



Scorpio, CJielifer, Phrynus, Phalangrium, Galeodes, are the 

 principal genera embraced by this division. 



2. The ARANEINA (or Spiders) have the abdomen not seg- 

 mented, and connected with the cephalothorax by a narrow 

 peduncle. The antennae are subchelate. The mandibular 

 palpi are filiform, their extremities being peculiarly modified 

 in the males. The two pair of maxillae are leg-like. 



Four or six conical papillae are situated on the ventral 

 surface of the abdomen, in front of the anus, and give exit to 

 the secretion of the silk glands. 



There are two or four pulmonary sacs, and two stigmata 

 connected with tracheae. 



3. The ACARINA (or Mites and Ticks) have the abdomen 

 unsegmented, and passing without any constriction into the 

 cephalothorax. 



The parts of the mouth are frequently converted into a 

 suctorial apparatus. When distinct respiratory organs are 

 present they take the form of tracheae. 



The foregoing are what may be called the typical Arachnida ; 

 the three following orders are aberrant forms^- 



1. The fresh- water ARCTISCA or TARDIGRADA (Water-bears), 

 are minute animals, which have no distinct abdomen, the 

 vermiform body representing the cephalothorax, which possesses 

 four pair of rudimentary limbs. The fourth pair are completely 

 posterior. 



The mouth is a suctorial tube, containing two stillets. 

 There are no respiratory or circulatory organs, and the sexual 

 apparatus is hermaphrodite. 



