AKACHNIDA. 141 



traordinary length, and contain prolongations from the stom- 

 ach. They are aU 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. 57^, a). 



Oedee it. AcAEDfA. — The most familiar members of this 

 order are the Mites and Ticks (Fig. 57^, i, 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|^, h) ; others are parasitic 

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

 adult (Fig. 51\, c) ; others are permanently parasitic upon 



Fig. 57X-— Arachnida. a Pycnoaonum UttoraU; 6 Tetramychus telarius, one of tlie 

 "Sociable" mites; c Hydraalma 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. 



Oedek 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 



