INVERTEBRATE ANIMALS. 81 



Class ARACHNIDA. apaxv>), a spider. 



The following characters may serve to distinguish the 

 animals of this class. Head and thorax amalgamated to 

 form a cephalo -thorax ; antennae none, or modified into 

 nippers ; legs never more than four pairs ; respiration 

 aerial ; transformations none, or indistinct. 



The authorities most used have been C. L. Koch 

 and Dr. C. W. Hahn, " Die Arachniden," 16 vols. 8vo., 

 and " Histoire Naturelle des Insectes Apteres," par M. 

 le Baron Walckenaer et M. Paul Gervais, 5 vols. 

 Svo., Paris, 1847. 



Group 194.— Sub-class ARACHNIDA-TRACHEARIA. 

 Respiration by the skin or by respiratory tubes, ti-achece. 



I. Order Podosomata. The Sea-spiders are found 

 on Fuci, or under stones near low-water mark on the 

 sea-coast, and are tardy creatures apparently made up 

 grotesquely of little more than jointed limbs. Pycno- 

 gonum, Nymphon, &c. 



II. Order Acarina. Water-bears, Tardigrada, 

 animalcules found in wet moss, &c. ; Mites, Acari ; 

 Water-mites, HydrachnidcB ; the Itch-acarus, Sarcop- 

 tes ; the harmless parasite on the face, Desmodex ; and 

 the Ticks, Ixodes. 



III. Order Adelarthrosomata. u^Kos, concealed ; 

 ap9(5ov, a joint ; (ra;/;ta, abody. Harvest-spiders, P/iaZaw- 

 gidce ; the Book-scorpion, Chelifer ; and the venomous 

 Galeodes of the family Solpugidce. 



The biology of this group, of which Walckenaer 

 describes about 450 species, when it shall have been 

 fully investigated must prove to be deeply interesting, 

 because of the extreme divergency amongst its con- 



