ARACHNIDA. 141 



traordinary length, and contain prolongations from the stom- 

 ach. They are all grotesque-looking animals, found at low 

 water upon stones or marine plants, or parasitically attached 

 to marine animals. One of the commonest forms is figured 

 below (Fig. 5Yi, a). 



ORDER II. ACARINA. The most familiar members of this 

 order are the Mites and Ticks (Fig. 57^, #, c). They are dis- 

 tinguished by the fact that the abdomen is amalgamated with 

 the cephalo-thorax to form a single mass. Respiration is ef- 

 fected by the general surface of the body or by air-tubes 

 (tracheae). 



The habits of the mites are extremely varied. Some are 

 found upon different plants (Fig. 57|-, b) ; others are parasitic 

 upon water-insects when young, but swim about freely when 

 adult (Fig. 57J, c) ; others are permanently parasitic upon 



FIG. 57X- Arachnida. a Pycnofjonum littorale; b Tetranyclms telariw, one of the 

 "Sociable" rnites; c Hydrachna globulus, one of the "Water-mites." 



other animals, such as sheep, dogs, insects, etc. ;- and others 

 inhabit decaying provisions, as is the case with the well-known 

 " cheese-mite " (Acarus domesticus). Two species have a con- 

 siderable medical interest as attacking man. One of these 

 causes the skin-disease which is known as the " itch," and the 

 other is found inhabiting certain glandular follicles of the 

 skin, probably without an exception even in favor of the most 

 cleanly people. 



ORDER III. PEDIPALPI. In this family are the most for- 

 midable of all the Arachnida namely, the Scorpions. They 

 are all distinguished by the fact that the abdomen is divided 

 into distinct segments, and is continued into the cephalo- 

 thorax without any well-marked boundary or constriction. In 



