ARACHNIDA. 



307 



secerning organs, conform more or less to the type of structure 

 met with in the class last described. The Mites (Acaridat) belong 

 to this division, and form a very numerous family, which is exten- 

 sively distributed. Some are parasitic in their habits, infesting 

 the bodies of insects ; and one, the itch-insect (Acarus Scabiei), 

 is found occasionally upon the human skin. Many live in cheese 

 and other provisions, where they multiply prodigiously ; and not 

 a few inhabit leaves, or are found under stones, or beneath the 

 bark of trees. Some (Hydrachna) are aquatic ; but unfortunately 

 in all, from their extremely minute size, the investigation of their 

 internal viscera presents so many difficulties, that but little is 

 satisfactorily known concerning their anatomy : even the pseudo- 

 Scorpionida, which are of larger growth, and, although still 

 breathing by tracheae, approximate most closely to the outward 

 form of the next group, hive been very imperfectly examined. 

 The rest of the Arachnidans breathe by means of lungs, or, as 

 they are more properly designated, pulmonary branchiae; and 

 consequently, in contradistinction to the last-mentioned, are called 

 by zoologists ARACHNIDA PULMONARIA : such are the Scor- 

 pions and Spiders. 



(Fig. 140.) 



The PEDIPALPI, forming the second division, are at once re- 

 cognised by the peculiarity of their external configuration. Their 

 palpi, the representatives apparently of the maxillary palpi of 

 insects, are exceedingly strong, and furnished at their extremity 

 with a prehensile forceps ; the hinder part of the body, correspond- 

 ing with the abdomen of insects, is much prolonged, and composed 



